sddm: unused tty do not expire

With KDM, when a tty remained unused it expired.
E.g. when I start a new session on tty8, but change my mind before actually login in, and go back to tty7, the tty8 login screen will remain there unused for ever.
If I then go to the K-menu and try to start another new session, the tty8 session is not reused. Instead we go to tty9.

KDM handled this more gracefully, by allowing unused tty sessions to expire after a certain time if 1) there was no successful login and 2) the user switched back to another session.