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.