Two new options:

- server option "exit-unattached" makes the server exit when no clients
  are attached, even if sessions are present;

- session option "destroy-unattached" destroys a session once no clients
  are attached to it.

These are useful for preventing tmux remaining in the background where
it is undesirable and when using tmux as a login shell to keep a limit
on new sessions.

server-fn.c

@@ -404,6 +404,25 @@ server_destroy_session(struct session *s)
 	recalculate_sizes();
 }
 
+void
+server_check_unattached (void)
+{
+	struct session	*s;
+	u_int		 i;
+
+	/*
+	 * If any sessions are no longer attached and have destroy-unattached
+	 * set, collect them.
+	 */
+	for (i = 0; i < ARRAY_LENGTH(&sessions); i++) {
+		s = ARRAY_ITEM(&sessions, i);
+		if (s == NULL || !(s->flags & SESSION_UNATTACHED))
+			continue;
+		if (options_get_number (&s->options, "destroy-unattached"))
+			session_destroy(s);
+	}
+}
+
 void
 server_set_identify(struct client *c)
 {