mount: use libmount to monitor mountinfo & utab

mount: use libmount to monitor mountinfo & utab

The current implementation directly monitor /proc/self/mountinfo and
/run/mount/utab files. It's really not optimal because utab file is
private libmount stuff without any official guaranteed semantic.

The libmount since v2.26 provides API to monitor mount kernel &
userspace changes and since v2.27 the monitor is usable for
non-root users too.

This patch replaces the current implementation with libmount based
solution.

Signed-off-by: Karel Zak <kzak@redhat.com>