services: Use (guix build syscalls) instead of util-linux.

* gnu/services/dmd.scm (dmd-configuration-file): Add derivations for
  the (guix build syscalls) module, and add that to the load path of
  dmd.conf.
* gnu/services/base.scm (root-file-system-service): Rewrite using the
  'sync' and 'mount' procedures.