libmount: make public top-level monitor FD only

libmount: make public top-level monitor FD only

We need full control on changes evaluation, so it's better to
hide all in our private epoll. This change also significantly
simplify the API.

 mn = mnt_new_monitor();
 mnt_monitor_enable_userapce(mn, TRUE, NULL);
 mnt_monitor_enable_kenrel(mn, TRUE);

 fd = mnt_monitor_get_fd(mn);
 ...
   <use 'fd' in epoll controlled by your application>
 ...
 while (mnt_monitor_next_changed(mn, &filename, NULL) == 0)
 	printf("%s: change detected\n", filename);

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