libmount: add utab.act file

libmount: add utab.act file

The file exists when libmount works with utab, and more steps are
expected during a single mount operation (more kernel events, more
updates to utab, etc.).

It is possible to monitor the file through the mnt_monitor_...() API
by a simple access() after any event. No locks are expected in
monitor, making it usable for non-root users without any security
impact. The monitor can ignore events if the file exist.

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