Suggested by Mark H. Weaver.
* gnu/system/shadow.scm (<user-group>)[system?]: New field.
(%base-groups): Introduce 'system-group' macro, and use it.
* gnu/system.scm (user-group->gexp): Pass the 'system?' field.
* guix/build/activation.scm (add-group): Add #:system? and honor it.
(activate-users+groups): Handle the 'system?' field.
* gnu/system/file-systems.scm (%tty-gid): Choose an ID below 1000.
* doc/guix.texi (User Accounts): Document the 'system?' field.
* gnu/system/file-systems.scm (<file-system>)[title]: New field.
* gnu/services/base.scm (file-system-service): Add #:title parameter.
In 'start' gexp, use 'canonicalize-device-spec' and honor TITLE.
* gnu/system.scm (other-file-system-services,
operating-system-root-file-system, operating-system-initrd-file):
Adjust accordingly.
* gnu/system/linux-initrd.scm (file-system->spec): Likewise.
* gnu/system/vm.scm (system-disk-image): Add 'title' field for the root
file system.
* guix/build/linux-initrd.scm (mount-file-system): Expect the second
element of SPEC to be the title.
(boot-system)[root-mount-point?, root-fs-type]: Likewise.
* gnu/services/dmd.scm (dmd-configuration-file): Select
'canonicalize-device-spec'.
This fixes a circular dependency between (gnu system) and (gnu system
linux-initrd), where the latter could end up being compiled before
'file-system-type' was defined as a macro.
* gnu/system.scm (<file-system>, %fuse-control-file-system,
%binary-format-file-system): Move to...
* gnu/system/file-systems.scm: ... here. New file.
* build-aux/hydra/demo-os.scm, gnu/system/linux-initrd.scm,
gnu/system/vm.scm: Use it.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.
Ludovic Courtès