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 --combined src/login1.c
index 991ebb08ab0dbb1eaf7839d474dcc524e61693b7,dc725067f4156bec09ee603619f0fe57d671ada3..1616073bfced0844a0348cb124f89c516ba10e5e
--- 1/src/login1.c
--- 2/src/login1.c
+++ b/src/login1.c
@@@ -269,7 -269,6 +269,6 @@@ add_seat (Login1Service *service, cons
                  seat->priv->can_graphical = g_variant_get_boolean (value);
              else if (strcmp (name, "CanMultiSession") == 0 && g_variant_is_of_type (value, G_VARIANT_TYPE_BOOLEAN))
                  seat->priv->can_multi_session = g_variant_get_boolean (value);
-             g_variant_unref (value);
          }
          g_variant_iter_free (properties);
          g_variant_unref (result);
@@@ -348,8 -347,8 +347,8 @@@ login1_service_connect (Login1Service *
                                                                     NULL,
                                                                     G_DBUS_SIGNAL_FLAGS_NONE,
                                                                     signal_cb,
 -                                                                   service,
 -                                                                   NULL);
 +                                                                   g_object_ref (service),
 +                                                                   g_object_unref);
  
      result = g_dbus_connection_call_sync (service->priv->connection,
                                            LOGIN1_SERVICE_NAME,