Instead of a buffer size limit on each pane, set a limit of 300 seconds

of data for each client in control mode.

tmux.h

@@ -1668,6 +1668,7 @@ struct client {
 	}		 exit_type;
 	enum msgtype	 exit_msgtype;
 	char		*exit_session;
+	char		*exit_message;
 
 	struct key_table *keytable;