dlm_controld: output explicit info about stateful merging
    
    When there are 3 or more partitions that merge, none may see enough
    clean nodes. Therefore, DLM would be stuck there forever until
    administrator manually reset/restart enough nodes to produce sufficient
    clean nodes. Therefore, output explicit information for higher code
    about the stateful merging state. Now, higher code can use `dlm status
    -v` to get "stateful_merge_wait". If it equals "1", we know dlm is
    waiting for manual intervention. Then, higher code can choose nodes
    to fence. DLM will continue to work if "clean nodes >= stateful merged
    nodes" becomes true.
    
    Signed-off-by: Eric Ren <zren@suse.com>