mntent-based implementation filter out mounts with device path that was
repeated. Consequently, it is not possible to show such mounts in UI even
with x-gvfs-show, because they are not returned from g_unix_mounts_get.
libmount-based implementation currently doesn't filter out any mounts
which causes issues to our volume monitors. Let's rather mark mounts
which don't point into fs root as system_internal. This approach won't be
affected by mount order as is mntent-based implementation. It will mark
more mounts as system_internal than it is filtered out with mntend-based
implementation, but there will be always possibility to show them in UI
over x-gvfs-show, which was not possible with mntend-based. We can
probably introduce some improvements later to not mark unique mounts as
system internal even if they don't point into fs root...

#1271