If systemd is not built with PAM support, systemd-user-sessions.service
won't be built. On systems without PAM, /run/nologin is useless. On
systems with PAM but systemd is not built with PAM, /run/nologin won't
be removed and all unprivileged users can't login.
So, we should not create /run/nologin if systemd is built without PAM.