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-client.c

@@ -185,6 +185,7 @@ server_client_lost(struct client *c)
 	c->flags |= CLIENT_DEAD;
 
 	recalculate_sizes();
+	server_check_unattached();
 	server_update_socket();
 }