services: Add service to cleanly unmount the root file system.

* gnu/services/base.scm (root-file-system-service,
  user-processes-service): New procedures.
  (mingetty-service, nscd-service, syslog-service, guix-service): Add
  requirement on 'user-processes'.
  (%base-services): Add (user-processes-service)
  and (root-file-system-service).
* gnu/services/xorg.scm (slim-service): Add requirement on
  'user-processes'.