user-notification: 'productify' the user-warning system

* Rename user_warning to user_notification
* Add warning and error types (in addition to the existing deprecated)
* Simplify logic when emitting a user notification after forking; we
  don't need to copy the notification data since we're in a new
  process and have total control over that memory.

terminal.c

@@ -944,7 +944,7 @@ term_init(const struct config *conf, struct fdm *fdm, struct reaper *reaper,
     if ((term->slave = slave_spawn(
              term->ptmx, argc, term->cwd, argv,
              conf->term, conf->shell, conf->login_shell,
-             &conf->warnings)) == -1)
+             &conf->notifications)) == -1)
     {
         goto err;
     }