libmount: improve monitor to be usable for non-root users

libmount: improve monitor to be usable for non-root users

The current implementation calls mkdir and open(O_CREATE) to
initialize /run/mount/utab.lock before it starts to monitor the file.
Unfortunately it makes the monitor useless for non-root processes
(e.g. systemd --user).

The new implementation adds inotify watch for the last existing
component in the path (/run/mount/utab.lock) and re-initialize
after a change. It makes the monitor robust enough for mkdir/rmdir
when monitor is already active.

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