Sync OpenBSD patchset 1069:

Add notify hooks for various events, the functions are currently empty
stubs but will be filled in for control mode later. From George Nachman.

cmd-rename-session.c

@@ -64,6 +64,7 @@ cmd_rename_session_exec(struct cmd *self, struct cmd_ctx *ctx)
 	RB_INSERT(sessions, &sessions, s);
 
 	server_status_session(s);
+	notify_session_renamed(s);
 
 	return (0);
 }