Fixes a longstanding issue whereby, due to our long fstab that included
pseudo file systems like cgroup mounts, graphical file managers would
display all of these. Initially reported at
<https://lists.gnu.org/archive/html/help-guix/2017-11/msg00084.html>.
* gnu/services/base.scm (file-system-fstab-entries): New procedure.
(file-system-service-type): Use it to extend FSTAB-SERVICE-TYPE.