Sometimes during startup, logind will send a `SeatNew` signal for
seat0 after GDM has already called `ListSeats` and processed `seat0`.

That `SeatNew` signal leads to GDM calling `create_display` twice in
quick succession.

This commit changes GDM to avoid such double processing, by ignoring
the `create_display` requests for seats that already have a prepared
display ("prepared" means "starting up").

Closes: #410