mgetty: use liblockdev for device locking by default
Former implementation of device locking was implemented directly
by mgetty. Now we will rely on liblockdev to do device locking.
Using liblockdev we solve issues described in RHBZ #754235, thus
device lockfiles are created in /var/lock/lockdev and it makes
mgetty interoperable with other software already migrated to
liblockdev, e.g. minicom.