services: Add 'file-system-service'.

* gnu/services/base.scm (file-system-service): New procedure.
  (user-processes-service): Add 'requirements' parameter.
* gnu/services/dmd.scm (dmd-configuration-file): Use (guix build
  linux-initrd).
* guix/build/linux-initrd.scm (guix): Export 'check-file-system'.
* gnu/system.scm (file-union): New procedure.
  (essential-services): Use it.  Add that to the returned list.