gunixmounts: Stop considering cifs/nfs as system file systems

They’re network file systems, but not system file systems (in the sense
that procfs is a system file system). This fixes them disappearing from
the sidebar in the UI.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

GNOME/glib#1424
6 jobs for cifs-is-not-a-system-fs in 44 minutes and 5 seconds (queued for 4 seconds)