system: File system sources can be marked as labels or devices.

system: File system sources can be marked as labels or devices.

* 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'.