libmount: update utab after successful extern helper mount

libmount: update utab after successful extern helper mount

This patch allows to maintain private utab libmount file also for
external mount/umount helpers that are not linked with libmount.

The libmount check if utab has been updated after successful extern
helper execution (status=0). If not then the file is updated.

This patch affects only 'user' fstab mount option. So, for example
with suid mount.cifs you can use:

 //server/foo    /mnt  cifs   username=foo,noauto,user

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