Fixes <http://bugs.gnu.org/22176>.
Reported by Florian Paul Schmidt <mista.tapas@gmx.net>.
* gnu/system/file-systems.scm (<file-system>)[mount?]: New field.
(file-system->spec): Adjust accordingly.
* gnu/services/base.scm (file-system-dmd-service): Return the empty list
when FILE-SYSTEM has 'mount?' set to false.
(user-processes-service): Select the subset of FILE-SYSTEMS that matches
'file-system-mount?'.
* doc/guix.texi (File Systems): Document it.
be21979d85