Use logind to provide the list of seats to use.

This deprecates the old method of defining seats in configuration. The xdg-seat option and the AddSeat() D-Bus method are now deprecated.
If logind is not present, only one seat will be started (seat0). If there is a use-case for non-logind multi-seat then we can look at creating something like /etc/lightdm/seats.conf as a fallback mechanism.

diff --cc src/login1.c
Simple merge