Rename option "node check --is-shutdown" to "--is-shutdown-cleanly"

As that's what we really want to know. Also return "UNCLEAN_SHUTDOWN"
if that's the case, rather than "RUNNING" which is confusing, even
though it's a command for internal use.

dbutils.h

@@ -54,7 +54,8 @@ typedef enum {
 typedef enum {
 	NODE_STATUS_UNKNOWN = -1,
 	NODE_STATUS_UP,
-	NODE_STATUS_DOWN
+	NODE_STATUS_DOWN,
+	NODE_STATUS_UNCLEAN_SHUTDOWN
 } NodeStatus;
 
 typedef enum {