sanlock: add ORPHAN_ONLY flag for acquire
    
    The existing ORPHAN flag will acquire an orphan lock
    if it exists, otherwise will acquire a new lock.
    
    The new ORPHAN_ONLY flag will acquire an orphan lock
    if it exists, but will not acquire a new lock.
    
    Signed-off-by: David Teigland <teigland@redhat.com>