login: set pos-slot to fallback on pos-eviction

login: set pos-slot to fallback on pos-eviction

If we evict a session position, we open the position slot for future
sessions. However, there might already be another session on the same
position if both were started on the same VT. This is currently done if
gdm spawns the session on its own Xserver.

Hence, look for such a session on pos-eviction and claim the new slot
immediately.