gunixmounts: Use libmnt_monitor API for monitoring

The GUnixMountMonitor object implements monitoring on its own currently. Only the /proc/mounts file changes are monitored. It is not aware of the /run/mount/utab file changes. This file contains the userspace mount options (e.g. x-gvfs-notrash, x-gvfs-hide) among others. There is a problem when /sbin/mount.<type> (e.g. mount.nfs) helper programs are used. In that case, the /run/mount/utab file is updated later than the /proc/mounts file and thus the GUnixMountMonitor clients (e.g. gvfs-udisks2-volume-monitor, gvfsd-trash) don't see the userspace options until the next mount-changed signal. Let's use the libmnt_monitor API for monitoring instead and emit the mount-changed signal also when the /run/mount/utab file is changed.

Related: https://issues.redhat.com/browse/RHEL-14607 Related: https://github.com/util-linux/util-linux/pull/2607