CONVERTIBLE HYDRATION SYSTEM Field of the Disclosure The present disclosure is directed generally to personal hydration systems, and more particularly to hydration systems adapted to provide variable functionality. Background of the Disclosure As used herein, the term "hydration system" refers to a fluid reservoir from which an elongate drink tube extends and terminates at a mouthpiece from which a user may draw drink fluid from the reservoir. The reservoir is often a flexible fluid reservoir that includes a resealable fill port through which drink fluid may be poured into the reservoir, and an exit port through which drink fluid may be drawn through the drink tube. The reservoir is typically housed in a body-mounted pack that enables the reservoir to be carried on a user's body. Conventional hydration systems include back-mounted backpack-style hydration systems, waist-mounted hydration systems, and hydration systems that include both waist and shoulder straps. Summary of the Disclosure The present disclosure is directed to personal hydration systems that are adapted to provide variable, or user-adaptable, functionality, such as to selectively configure the systems for a variety of applications. The hydration systems include a fluid reservoir that is adapted to receive and contain a volume of drink fluid. An elongate downstream assembly extends from the reservoir and enables a user to draw drink fluid from the reservoir, such as by sucking upon a mouthpiece that may form a portion of the downstream assembly. The downstream assembly may include a plurality of fluidly interconnected components, and typically will include at least an exit port that fluidly interconnects the downstream assembly with the reservoir, at least one length of drink tubing through which the drink fluid may flow, and a mouthpiece or other outlet from which the drink fluid may be dispensed from the hydration system. The reservoir, and typically a portion of the downstream assembly, is housed within a pack. The pack includes a strap assembly with at least one body-mounting strap, such as a pair of shoulder straps. Unlike conventional packs, the present system includes a pack with selectively variable functionality. The pack may include stowable straps that may be selectively housed within the pack. The pack may include attachment mechanisms that are adapted to selectively attach the pack (with stowed or deployed straps) to other body-mounted articles, such as another pack, a user's clothing, and/or a web gear or other body harness. The pack may include a plurality of object-securing regions that are designed to receive and/or couple other objects to be transported to the pack. The straps may be configured to be compatible with both the above-discussed stowability and the potential increased load when other objects are secured to the pack. The hydration system may additionally or alternatively include component connectivity, which provides one or more quick-connect assemblies adapted to enable quick and repeated interchange of functional, fluidly interconnected components in the hydration system's downstream assembly. In some embodiments, the hydration system includes a quick-connect assembly that is adapted to selectively couple a bite- actuated mouthpiece and a gas mask adapter to the hydration system's drink tube. The hydration system may be at least partially, if not completely, formed from chemically resistant materials, such as to provide resistance to chemical agents such as sarin nerve agent and mustard or other blister agents. Brief Description of the Drawings Fig. 1 is an isometric view of a hydration system according to the present disclosure. Fig. 2 is a front elevation view of the hydration system of Fig. 1. Fig. 3 is a front elevation view of the hydration system of Fig. 2 with the strap assembly in a stowed configuration. Fig. 4 is a rear elevation view of the hydration system of Fig. 1. Fig. 5 is a fragmentary view of an illustrative attachment mechanism suitable for use with hydration systems according to the present disclosure. Fig. 6 is a fragmentary view of another illustrative attachment mechanism suitable for use with hydration systems according to the present disclosure. Fig. 7 is a somewhat schematic assembly view illustrating examples of structures to which convertible hydration assemblies according to the present disclosure may be coupled, such as when the strap assemblies of the hydration assemblies are in a stowed configuration. Fig. 8 is an isometric view of an illustrative hydration assembly. Fig. 9 is a top plan view of another illustrative hydration assembly. Fig. 10 is a top plan view of another illustrative hydration assembly. Fig. 11 is a cross-sectional view of an illustrative bite-actuated mouthpiece in a closed configuration. Fig. 12 is a cross-sectional view of an illustrative bite-actuated mouthpiece in a dispensing configuration. Fig. 13 is a fragmentary rear elevation view of the hydration system of Fig. 1. Fig. 14 is an elevation view of a hydration system that includes a quick- connect assembly and which is fluidly interconnected with a gas mask. Fig. 15 is an elevation view showing an illustrative quick-connect kit. Detailed Description and Best Mode of the Disclosure A personal hydration system according to the present disclosure is shown in Fig. 1 and generally indicated at 10. As shown, hydration system 10 includes a pack 12 and a hydration assembly 14, which is at least partially received within a compartment 16 within the pack. More specifically, the hydration assembly includes a fluid reservoir that is housed within compartment 16 and an elongate downstream assembly 20 that extends in fluid communication with the reservoir from the compartment and terminates at a mouthpiece or other outlet from which water or other potable drink fluid may be drawn from the reservoir. Compartment 16 may be referred to as a reservoir compartment, in that it is configured to receive the fluid reservoir of the hydration assembly. As discussed in more detail herein, downstream assembly 20 includes at least an elongate drink tube that extends in fluid communication with the reservoir and through which drink fluid may be selectively drawn from the reservoir by a user. The drink tube may include an end region to which a mouthpiece is mounted or otherwise present. Hydration assembly 14 is discussed in more detail herein, such as with respect to Figs. 7-15. Pack 12 includes a pack body 30 that includes forward and rearward surfaces 32 and 34, with forward surface 32 referring generally to the surface, or portion, of the pack that faces and/or contacts a user's body when hydration system 10 is worn on a user's body, such as when strapped on a user's torso, secured to a user's clothing, or otherwise worn as described herein. Rearward surface 34 refers generally to the surface, or portion, of the pack that faces generally away from the user's body when the hydration system is worn or otherwise secured to a user's body. As used herein, the terms "forward surface" and "rearward surface" are intended to generally refer to the forward and rearward regions of the pack body, with it being within the scope of the present disclosure that these regions may be comprised of two or more surfaces, materials, etc., and that these regions are not required to be smooth, flat, continuous, etc. The forward surface may also be described as being a front surface, or a body- facing surface, and the rearward surface may also be described as being a rear surface or a surface adapted to face away from a user's body when the pack is supported on a user's body. Forward surface 32 of pack body 30 is shown in Fig. 2. Surface 32 is preferably formed from a flexible material, and may optionally include, or be, at least partially formed from a padding material to cushion the engagement of the pack against the user's back or other body portion. Pack body 30 and/or forward surface 32 may, but are not required to, include a frame. When present, the frame may be utilized to support and/or define the shape of the pack body. Illustrative, non- exclusive examples of frames for hydration systems are disclosed in U.S. Patent No. 6,892,915, the complete disclosure of which is hereby incorporated by reference for all purposes. As indicated above, it is also within the scope of the present disclosure that the hydration system does not include a structural frame, such as may be formed from metal, molded plastic, or other suitable materials. Also shown in Fig. 2 is a harness assembly 40, which is designed to selectively mount the pack for carrying by a user, such as on a user's back. In the illustrated example shown in Fig. 2, the harness assembly includes a pair of strap assemblies 42 that are selectively adapted to extend around a portion of a user's body, such as the user's shoulders, to support the pack thereupon. When used to directly secure the pack to a user's body, the strap assemblies may be referred to as body- securing strap assemblies in that they are configured to define a closed loop around a portion of a user's body, with the closed loop defined either entirely by the strap assembly, or by the strap assembly in combination with other elements of the pack, such as the pack body. For example, in Fig. 2, harness assembly 40 includes a pair of strap assemblies 42 in the form of shoulder strap assemblies 44 and 46. Each shoulder strap assembly defines a closed loop through which a user's arm and shoulder may be inserted so that the shoulder strap assemblies secure and retain the pack on the user's back. The strap assemblies may include padded regions, such as to cushion the engagement of the hydration system with a user's body. When the strap assemblies are configured and positioned to secure the pack on a user's back or other body portion, the strap assemblies may be described as being in a deployed, or body-securing configuration. In such a configuration, the strap assemblies may be described as defining with the pack closed loops that are sized to receive a user's arms therethrough to support the hydration system on a user's back. As discussed in more detail herein, it is within the scope of the present disclosure that the hydration system's strap assemblies may be selectively configured between the deployed configuration and a stowed configuration, in which the strap assemblies are retained within and/or against the pack body and thereby not positioned and/or not configured to receive a user's arms therethrough to secure the pack on a user's back. In the illustrative example shown in Figs. 1 and 2, each strap assembly includes upper and lower strap members 48 and 50 that are either fixedly connected or releasably connected to the pack. As used herein, the term "connected" includes either direct uninterrupted attachment or connection via an intermediate structure. For example, an end region of a strap member may be connected to the pack via a hip belt, D-ring, strap loop, clip, fastener, pack extender, or other intermediate structure. Similarly, as used herein, "fixedly connected" refers to mechanisms of securing an end region of a strap assembly to the pack body in a manner so that the end region may not be released or otherwise removed from its connection with the pack body without destroying at least a portion of the strap assembly, pack body, or intermediate structure that fixedly connects the two. In contrast, "releasably connected" refers to mechanisms for interconnecting an end region of a strap member with the pack body so that the end region is adapted to be repeatedly disconnected from and reconnected to the pack body. Illustrative examples of mechanisms for fixedly connecting an end region of a strap member include sewing the end region to, or around, the pack body or to a fixedly connected intermediate structure. Another example is to form the strap member from the same continuous length of material as the portion of the pack body to which it is fixedly connected. Examples of mechanisms for releasably connecting an end region to the pack body include buckles, clips, knots, snaps, locks, and other releasable fasteners. In the illustrative embodiment shown in Figs. 1 and 2, the strap members are adjustably secured together, such as to permit the size of the closed loop defined by the strap assembly and the pack body to be adjusted. For example, selectively resizing the closed loop may permit the hydration system to be sized for different sized users and/or to adjust the position of the hydration system relative to a particular user's body, such as by adjusting the relative vertical placement of the pack body relative to the user's body. As perhaps best seen in Fig. 2, the strap assemblies include a coupling member 52 that adjustably interconnects the strap members to define a closed loop with the pack body. As shown, strap member 50 is adjustably threaded through the coupling member, and includes a free end, or terminal length, 54 that extends from the coupling member and does not form part of the closed loop. This terminal length may hang loosely from the coupling member. In the illustrative example, the strap assemblies include strap management systems 56 that are adapted to secure the terminal length of the free end of the strap assemblies relative to the rest of the pack, such as to bundle and/or otherwise retain the terminal length proximate the coupling member and/or the rest of one of the strap members. In the illustrated example, the strap management system includes a pair of members 57 that project from the free end region and are adapted to be releasably secured together, such as to secure the unbundled free end region against another portion of the strap assembly and/or to secure the free end region in a bundle, such as by after winding it around itself. Illustrative examples of strap management systems are disclosed in U.S. Patent Application Serial No. 10/185,428, the complete disclosure of which is hereby incorporated by reference for all purposes. It is within the scope of the disclosure that the strap assemblies may be formed from a single strap member that is adjustably or fixedly secured relative to the pack body to define a closed loop. It is further within the scope of the present disclosure that strap members 48 and 50 are releasably, but not adjustably, coupled together, or that they are adjustably, but not releasably, coupled together. Unlike conventional hydration systems in which any coupling member includes only a slidable adjustment member, or ladder-lock, 58, coupling member 52 further includes a quick-release mechanism 60 that enables the strap members to be quickly and repeatedly disconnected and reconnected to each other simply by disconnecting and reconnecting the mating portions 62 and 64 of mechanism 60. In the illustrative embodiment shown in Fig. 2, portions 62 and 64 may be described as male and female portions or clip members. Although it is within the scope of the present disclosure that hydration system 10 may be formed with conventional coupling members, or no coupling member at all, utilizing a quick-release mechanism for the strap assemblies adapts the assemblies for easy storage and/or removal. For example, when the strap members are disconnected from each other, the strap members may be selectively stowed within a compartment of the pack, as described in more detail herein. Also shown in Fig. 2 is an optional sternum strap assembly 70, which selectively draws the strap assemblies together. As illustrated, the sternum strap assembly includes strap members 72 and 74, which are adjustably and releasably coupled together by another coupling member 52. Similarly, the entire sternum strap assembly is slidably adjustable within a range of positions along strap members 48 by clips 76 that are configured to slide along guides 78. The illustrated clips are configured to be selectively removed and replaced relative to guides 78, although non-removable and/or non-adjustable clips may be used as well. In Fig. 2, pack body 30 is shown including at least one strap-stowage compartment 66 with openings 68 through which the strap members may be selectively inserted into the compartment and stowed until it is desirable to remove and reconnect the strap members to define the body-securing closed loop described above. For example, and as discussed in more detail herein, it is within the scope of the present disclosure to configure the strap assemblies to their stowed configuration, such as by disconnecting coupling members 52, with the hydration assembly thereafter being secured by the subsequently described coupling members to another structure, such as another pack, a vest, a harness, or an article of clothing with complimentary positioned mounts or engagement members for the coupling members. Compartment 66 may also be referred to as a strap compartment. In Fig. 3, hydration system 10 is shown with its strap assemblies stowed, such as in compartment 66, as opposed to the deployed configuration shown in Figs. 1 and 2. This configuration may be referred to as the stowed configuration of the strap assemblies. Compartment 66 may be coextensive with compartment 16, interconnected with compartment 16, or separate from compartment 16 without departing from the scope of the present disclosure. Similarly, compartment 66 may define subcompartments, or regions, for each strap assembly and/or each strap member. It is within the scope of the present disclosure that the strap assemblies may each include more than one quick-release mechanism, such as to enable the entire strap member(s) to be selectively disconnected and reconnected to the pack body. When an entire strap member is disconnected from the pack body, it may be stowed in any convenient location, either on the pack body or elsewhere. Rearward surface 34 of pack body 30 is shown in more detail in Fig. 4. As shown in solid lines, a portion of a drink tube that forms a portion of downstream assembly 20 is shown extending from compartment 16 through an opening adjacent one of the pack's strap assemblies. Internal compartment 16 may include an opening adjacent both shoulder strap assemblies to permit a user to selectively position the drink tube over either of the user's shoulders. Also shown in Figs. 1-4 are a plurality of attachment mechanisms 80 that extend from the pack body and which are adapted to selectively secure the pack body to another structure and/or to secure other objects to the pack body. For example, attachment mechanisms 80 may be used to secure pack body 30, and thus the entire hydration system, to another pack, such as when a user is carrying a primary, larger pack and also wants to carry hydration system 30. Illustrative, non-exclusive examples include backpackers that are carrying camping equipment, soldiers that are carrying military packs, etc. As another example, attachment mechanisms may be used to secure system 10 to a body harness, such as web gear that is worn by soldiers, climbing harnesses that are worn by mountain climbers, etc. As yet another example, attachment mechanisms 80 may be used to secure system 10 directly to a user's clothing, such as if the clothing includes loops or other regions to which mechanisms 80 may be releasably secured. When mechanisms 80 are utilized to secure system 10 to another pack or directly to a user's clothing or other harness, strap members 48 and 50 are preferably stowed, such as within compartment 66. Attachment mechanisms 80 may also be referred to as coupling mechanisms, or coupling members. Attachment mechanisms 80 also may be individually and/or collectively used to secure other objects to hydration system 10. For example, the mechanisms may be used to secure objects that do not themselves include clips or other fasteners for securing these objects to the hydration system. Illustrative, and by no means exclusive, examples of these objects include tools, camping equipment, radios, communication equipment, weapons, sporting goods, hunting equipment, eyewear, navigation equipment, marking equipment, etc. As indicated in at least Figs. 1 and 4, mechanisms 80 each include an anchor member 82, which is connected to the pack body and defines a passage 84. The mechanisms each also include a clasp 86, which is connected to the pack body by a strap member 88 and which is selectively inserted through the passage to secure the attachment mechanism around structure that is bounded by a closed loop formed by the strap member, clasp, anchor member, and/or pack body. In Fig. 5, attachment mechanism 80 is shown forming such a closed loop, which is indicated at 200 and is shown extending around another object, which is schematically illustrated and indicated at 202. As shown in Fig. 5, passage 84 and clasp 86 are configured so that the clasp may be inserted through the passage by selective positioning of the clasp to orient the clasp for insertion through the passage. After insertion through the passage, the clasp is adapted to be restrained from being withdrawn through the passage until intentionally repositioned by a user to reorient the clasp for passing back through the passage. As discussed, object 202 may take a variety of shapes, sizes, and structures, such as forming a portion of an object to be secured to and carried with hydration system 10 or forming a portion of a mounting loop or other mounting structure to which the attachment mechanism may be releasably secured, such as when the hydration assembly's strap assemblies are in their stowed configuration. In some embodiments, it may be desirable for loop 200 to be sized to extend tightly around object 202, while in others it may be sufficient simply for loop 200 to form a closed perimeter around the object and thereby prevent removal of the object from the loop until a user intentionally removes clasp 86 from passage 84 to open the loop. In Fig. 5, clasp 86 and anchor member 82 are coupled to the pack's body 30 by separate strap members 88 and 89, respectively. It is within the scope of the present disclosure that these strap members may form portions of a continuous strap (such as may be folded or otherwise secured to the pack body at an intermediate region) or may consist of separate components. It is also within the scope of the present disclosure that either or both of these strap members may be formed from an elastic material and/or may have an adjustable length. It is within the scope of the disclosure that attachment mechanisms 80 may include separate strap members for each clasp and anchor member and/or that either or both of these members are otherwise secured to the pack body. As an illustrative example of such a construction, Fig. 6 provides an example of a attachment mechanism in which the clasp 86 and anchor member 82 are connected to the strap body by a single strap member 88 to which the clasp and anchor member are interconnected in a spaced-apart relationship. It is within the scope of the present disclosure that mechanisms 80 may be differently embodied. As such, it is within the scope of the present disclosure that the mechanisms may have any suitable structure for selectively and positively securing the hydration system to other objects and/or securing other objects to the hydration system. By "positively securing," it is meant that the attachment mechanisms are adapted to secure the hydration system to another object or to secure another object to the hydration system without requiring more than an opening or other passage through which the attachment mechanism may form a closed loop around and/or a sufficiently sized region that the attachment mechanism may extend tightly around. As a variant of this structure, the attachment mechanisms may be configured to include one portion of a clip or other releasable fastening mechanism, with such an embodiment of the mechanism being adapted to mate with particular objects, such as other packs, harnesses, etc., which include the other portion of the clip or other releasable fastening mechanism. As discussed, attachment mechanisms 80 may be adapted to secure the hydration system, typically with its strap assemblies in a stowed configuration, to another object, such as another (typically larger) pack, a body-worn harness, or an article of clothing. Fig. 7 provides a somewhat schematic graphical example of these illustrative configurations for convertible hydration systems 10 according to the present disclosure. As shown, system 10 is illustrated with its strap assemblies in their stowed configuration. Also shown in Fig. 7 are three illustrative (non-exclusive) examples of objects 230 to which hydration system 10 may be secured by attachment mechanisms 80. The illustrated examples include a body-worn harness 240, such as may be worn by such individuals as soldiers, construction works, mountain climbers, and the like, a garment 250, such as a shirt, vest, or jacket that is worn by a user, and a pack 260, such as a backpack that is secured on a user's body by shoulder straps 262. The illustrative examples of objects 230 may be referred to as body-carried objects in that they are all designed to be worn or otherwise secured on a user's upper torso. Each object includes a plurality of mounting structures 232 that are positioned to be received within the closed loops formed by the corresponding plurality of attachment mechanisms 80 on hydration system 10. The mounting structures 232 may have any suitable flexible or rigid and fixed or adjustable configuration suitable to be received within the closed loops provided by attachment mechanisms 80. The mounting structures may additionally or alternatively include one or more clasps and/or anchor members that correspond in size to the anchor members (and more particularly, the passages formed therein) and clasps of the attachment mechanisms to enable the corresponding structures to be secured together to secure the hydration system to the object. Figs. 1-2 and 4 also illustrate that hydration system 10 may include a plurality of object-securing regions 90 that define, or include, fixed loops to which other objects may be tied, clipped or otherwise secured. Also shown is an optional carrying handle 92, which also may function as an object-securing region. As discussed, hydration system 10 includes a fluid reservoir that is housed within a compartment 16 within the hydration system's pack. Compartment 16 is typically positioned between surfaces 32 and 34 of the pack body. The reservoir is selectively filled with a volume of potable drink fluid, such as water or a sports drink, and the pack body may be specifically sized to receive a fully charged fluid reservoir. In some applications, it may be desirable for the pack body to be as small as possible, and accordingly, the pack may not be designed to hold objects other than a fully charged fluid reservoir within the compartment. It is within the scope of the disclosure, however, that the pack may be designed to receive other objects into compartment 16 in addition to the reservoir and/or that the pack includes other compartments in addition to compartments 16 and 66, such as one or more storage pockets. Illustrative examples of hydration assemblies are shown in Figs. 8-10 and include a reservoir 100 from which an elongate downstream assembly 20 extends and through which drink fluid is selectively dispensed from the reservoir. An exit port 102 fluidly interconnects the reservoir with the downstream assembly. In the illustrated example, the downstream assembly is fluidly coupled to the exit port at a first end region 104 and includes at least one length of flexible tubing 106, which collectively or individually may be referred to as a drink tube. End region 104 may be fluidly coupled to the exit port via any suitable configuration, such as by being releasably mounted on exit port 102, fixedly or even integrally mounted on the exit port, or inserted through the exit port and into the internal chamber of the reservoir. A user draws drink fluid from a distal end region 108 of the downstream assembly, such as by sucking upon a mouthpiece 110. Accordingly, downstream assembly 20 should be sufficiently long that a user may comfortably draw drink fluid from the reservoir through the downstream assembly by sucking upon mouthpiece 110 when the hydration system is properly worn on a user's back or otherwise secured to a user's body, as described herein. Reservoir 100 may be formed from any suitable rigid and/or flexible material. Preferably, the reservoir is at least substantially, if not completely, formed from a flexible material that is itself either waterproof or which includes a waterproof liner or other layer. An example of a suitable material is polyurethane, although others may be used. Reservoir 100 is preferably sized to hold at least 20 fluid ounces of drink fluid, and typically will include at least 50 oz. Illustrative examples of suitable reservoirs include reservoirs that are sized to hold 20-200 oz., or more, of drink fluid, such as 50 oz., 64 oz., 80 oz., 100 oz., 150 oz., etc. As shown in Figs. 8-10, the downstream assembly terminates distal the reservoir at a mouthpiece 110. In the illustrated examples, mouthpiece 110 takes the form of a bite-actuated mouthpiece that is selectively configurable between a dispensing position, in which drink fluid may be drawn from the reservoir and through an outlet or opening in the mouthpiece, and a closed position, in which the opening is closed so that drink fluid cannot pass therethrough. In Figs. 11 and 12, an illustrative bite-actuated mouthpiece is shown in its closed and dispensing positions. A bite-actuated mouthpiece is a resilient, self-sealing mouthpiece that is biased to the closed position, and is adapted to be configured to the dispensing position by a user placing the mouthpiece in the user's mouth and biting upon the appropriate sidewalls of the mouthpiece. Illustrative examples of bite-actuated mouthpieces are disclosed in U.S. Patent Nos. 6,070,767, 6,032,831, and 6,364,168, the complete disclosures of which are hereby incorporated by reference for all purposes. It is within the scope of the disclosure that the hydration systems disclosed herein may be used with other bite-actuated mouthpieces, that the hydration systems may be used with mouthpieces that are not self-sealing mouthpieces and which therefore require manual manipulation between the dispensing and closed positions, and that the hydration systems may be used with mouthpieces that are integrated with the drink tube. Reservoir 100 is preferably adapted to be selectively refilled, such as through a sealable fill port 146, and is in fluid communication with drink tube 106. The fill port may be selectively sealed by any suitable closure mechanism 112. In the illustrated examples, the reservoirs are shown including closure mechanisms 112 in the form of caps 114 that are removably coupled to the fill port to selectively seal the fill ports, although any suitable mechanism for selectively sealing the fill ports of the reservoirs may be used without departing from the scope of the present disclosure. In Fig. 8, the cap is adapted to seal the fill port through a simple friction fit, while in Figs. 9 and 10, the caps are adapted to threadingly engage a corresponding set of threads on the fill ports. Any suitable closure mechanism and sealing mechanism(s) may be used. In Fig. 8, the cap is coupled to the fill port by an optional tether 116 that extends from the cap and around the fill port. Tether 116 does not provide support to the fill port but does retain the cap proximate the fill port. In Fig. 9, fill port 146 is shown being sealed by a removable cap 114, with the fill port being supported by a collar 118 that extends around the fill port and provides support thereto. Also shown in Fig. 9 is another example of a suitable tether 116, with the illustrated tether extending within the reservoir when the fill port is sealed by the cap. In Fig. 10, the fill port includes a support collar with a projecting handle 120. Handle 120 may be used to position the reservoir for filling, and also may function as a counter-lever as the user twists or otherwise manipulates the cap to remove or secure the cap relative to the fill port. Preferably, the handle of the collar extends in a fixed orientation or limited range of rotational orientations relative to the fill port. In at least Figs. 9-10, downstream assembly 20 is shown including an optional accessory in the form of a manually actuated on/off valve 132. On/off valve 132 enables a user to selectively prevent drink fluid from being able to be drawn through the reservoir regardless of the configuration of, or the forces being applied to, the mouthpiece. As shown, valve 132 interconnects end region 108 of the downstream assembly's drink tube and mouthpiece 110. It is within the scope of the disclosure that the valve may be otherwise positioned within the downstream assembly, such as between adjacent lengths of drink tubing and/or integrated with the mouthpiece or other components of the downstream assembly. Additional examples of suitable structures for the hydration assemblies, and components thereof, are disclosed in U.S. Patent Nos. 6,675,998, 6,070,767, and 6,032,831, as well as in U.S. Patent Application Serial No. 10/666,856, the complete disclosures of which are hereby incorporated by reference for all purposes. It is within the scope of the disclosure that fill port 146 may be only accessible for filling the reservoir when the reservoir is removed from compartment 16. In such an embodiment, the pack body does not include a fill port opening through which the fill port, and more specifically cap 114 or any other utilized closure mechanism 112, may be accessed to selectively remove the cap (i.e., to open/unobstruct the fill port). In Fig. 13, pack body 30 is shown including a fill port opening 144 through which the fill pott 146 of the reservoir of the hydration assembly extends. As shown, fill port opening 144 is formed in rearward surface 34 of the pack body and faces generally away from body-contacting surface 32. Although this orientation is not required, it may be desirable because it orients the fill port and any cap or other closure mechanism away from the user's body in a position where the cap or other closure mechanism will not be pressed against the user's body when system 10 is properly worn. In Fig. 13, pack body 30 is shown including a reservoir opening 152 through which the reservoir may be selectively removed from and reinserted into compartment 16 within the pack body. When pack body 30 includes such an opening, the opening may include a suitable closure mechanism, such as a zipper, hook-and-loop mechanism, snap(s), etc. In the illustrated embodiment, the pack includes a cover 150 that selectively extends over the fill port, such as to provide a dust cover. As shown in Figs. 1 and 4, the cover selectively folds over the reservoir opening and the fill port, with the cover selectively retained in this position by at least one closure mechanism 154. In Fig. 13, illustrative examples of suitable closure mechanisms 154 are shown, such as zippers 156 and a hook-and-loop mechanism 158. Fig. 13 also demonstrates a reservoir that has a support collar with a differently sized projecting handle 120. It is within the scope of the present disclosure that the reservoir is fixedly secured within the pack's compartment. By this it is meant that the pack does not include a reservoir opening that is sized to permit repeated removal and reinsertion of the reservoir from the pack's compartment. Similarly, the pack may be formed without cover 150. An example of such a pack is shown in Fig. 14. Fig. 14 also demonstrates that downstream assembly 20 may (but is not required to) include at least one quick-connect assembly 170 that selectively enables the components of the downstream assembly to be removed and replaced or otherwise interchanged. Assembly 170 is adapted to fluidly and mechanically interconnect portions of the hydration system downstream (toward the mouthpiece, distal end region, etc.), relative to reservoir 100. Assembly 170 enables the interconnected components to be quickly and repeatedly coupled together and released from engagement without requiring the time or effort required with conventional hydration system components. As such, the quick-connect assembly may also be described as a quick connect/disconnect assembly, or quick coupling assembly. For example, the quick-connect assembly may include at least a pair of members that are configured to be fluidly connected with adjacent components of a hydration system. The members are further adapted to selectively and releasably interconnect with each other, such as by being releasably secured together by a lock member of the assembly. Illustrative examples of quick-connect assemblies 170, optional components of downstream assembly 20, and optional constructions, such as chemically-resistant constructions, for reservoir 100 and/or downstream assembly 20 are disclosed in U.S. Patent Application Serial No. 10/666,856, the complete disclosure of which is hereby incorporated by reference for all purposes. In Fig. 14, quick-connect assembly 170 is shown fluidly interconnecting drink tube 106 with a gas-mask adaptor 160, namely, a fitting that is adapted to be connected to a gas mask 190. Gas mask 190 is somewhat schematically illustrated and is intended to graphically represent any suitable gas mask, such as may cover a user's face, and gas masks that cover a user's entire head. Regardless of the configuration, mask 190 is adapted to provide drink fluid from reservoir 100 to the user's mouth without exposing the drink fluid to the environment outside of the hydration system and gas mask. In the illustrated embodiment, downstream assembly 20 may be described as including a length 106 of flexible tubing that fluidly interconnects the exit port of the hydration system's reservoir with quick-connect assembly 170 and a length 194 of tubing that fluidly interconnects assembly 170 and gas mask 190. Length 194 may be the intake tube of the gas mask or may be fluidly interconnected with the intake tube of the gas mask. Each of these lengths of tubing may be comprised of one or more fluidly interconnected tube portions. When a user desires to use the hydration system of Fig. 14 without a gas mask, the quick-connect assembly may be released, and a mouthpiece, or mouthpiece fitting, may be connected to the quick-connect assembly. When hydration system 10 includes one or more quick-connect assemblies, it preferably includes a plurality of interchangeable components, which may be referred to as a quick-connect assembly kit. For example, a portion (such as a male or female portion) of the quick connect assembly may be adapted to be connected to the distal end of a length of drink tubing. This portion of the quick-connect assembly is adapted to releasably interconnect and establish fluid communication with a corresponding (female or male) portion, such as may be integrally or otherwise associated with a variety of downstream components, such as mouthpieces, gas mask adapters, on/off valves, fittings for other components, etc. The selective interconnection of a variety of components via a quick-connect assembly is illustrated in Fig. 15. As illustrated, assembly 170 includes female member 180 that selectively interconnects and establishes fluid communication with a plurality of male members 182. The female member includes a lock mechanism, or member, 184 that releasably engages and prevents unintentional removal of the male member until a user actuates a release 212, such as depressible button 214. Industrial Applicability The hydration systems of the present disclosure are applicable to the hydration field and to the load-carrying pack fields. It is believed that the disclosure set forth above encompasses multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific examples thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. The subject matter of the disclosures includes all novel and non-obvious combinations and subcombinations of the various elements, features, functions and/or properties disclosed herein. Similarly, where the claims recite "a" or "a first" element or the equivalent thereof, such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. It is believed that the following claims particularly point out certain combinations and subcombinations that correspond to disclosed examples and are novel and non-obvious. Other combinations and subcombinations of features, functions, elements and/or properties may be claimed through amendment of the present claims or presentation of new claims in this or a related application. Such amended or new claims, whether they are directed to different combinations or directed to the same combinations, whether different, broader, narrower or equal in scope to the original claims, are also regarded as included within the subject matter of the present disclosure.