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.

repmgr-client-global.h

@@ -80,7 +80,7 @@ typedef struct
 	bool		siblings_follow;
 
 	/* "node status" options */
-	bool		is_shutdown;
+	bool		is_shutdown_cleanly;
 
 	/* "node check" options */
 	bool		archive_ready;