repmgr: catch possible corner case when checking node shutdown status

It's conceivable that PQping is returning "no response" but the
shutdown hasn't quite completed.

dbutils.h

@@ -74,6 +74,7 @@ typedef enum
 {
 	NODE_STATUS_UNKNOWN = -1,
 	NODE_STATUS_UP,
+	NODE_STATUS_SHUTTING_DOWN,
 	NODE_STATUS_DOWN,
 	NODE_STATUS_UNCLEAN_SHUTDOWN
 } NodeStatus;