SDDM 0.20.0-1ubuntu1 is broken for multi-seat setups in (K)Ubuntu 23.10

If you try to use sddm to login on a "second seat", you will see that nothing is initialised properly.

For example:

* Directory /run/user/1001 is not created

* DBUS_SESSION_BUS_ADDRESS of the second user is empty

* systemctl --user list-units prints
  Failed to connect to bus: No medium found

* Finally, loginctl session-status says:
  Could not get properties: Unknown object 'org/freedesktop/login1/session/auto'

In the logs of sddm, there is:

pam_systemd(sddm:session): Failed to create session: TTY tty1 is virtual console but seat seat1 is not seat0

There are at least 4 reports of the (same?) issue on https://github.com/sddm/sddm:

* Jul 14: https://github.com/sddm/sddm/issues/1762 (SDDM 0.20.0 fails to create session in multi-seat configuration)
* Jul 23: https://github.com/sddm/sddm/issues/1769 (SDDM doesn't seem to handle starting Wayland sessions on seats without VTs well)
* Aug 3: https://github.com/sddm/sddm/issues/1778 (Wayland multiseat)
* today: https://github.com/sddm/sddm/issues/1786