Make handshake between primary and backup controller cleaner.

src/slurmctld/backup.c

@@ -357,6 +357,7 @@ static int _background_process_msg(slurm_msg_t * msg)
 		} else if (super_user && 
 			   (msg->msg_type == REQUEST_CONTROL)) {
 			debug3("Ignoring RPC: REQUEST_CONTROL");
+			error_code = ESLURM_DISABLED;
 		} else {
 			error("Invalid RPC received %d from uid %u", 
 			      msg->msg_type, uid);

src/slurmctld/controller.c

@@ -1051,11 +1051,14 @@ static int _shutdown_backup_controller(int wait_time)
 		error("_shutdown_backup_controller:send/recv: %m, %s", TIME_STR);
 		return SLURM_ERROR;
 	}
-	if (rc) {
+	if (rc == ESLURM_DISABLED)
+		debug("backup controller responding");
+	else if (rc == 0)
+		debug("backup controller has relinquished control");
+	else {
 		error("_shutdown_backup_controller: %s", slurm_strerror(rc));
 		return SLURM_ERROR;
 	}
-	debug("backup controller has relinquished control");
 
 	/* FIXME: Ideally the REQUEST_CONTROL RPC does not return until all   
 	 * other activity has ceased and the state has been saved. That is