Logging into Unity8 session fails with gdm

[AlanGriffiths]

This was independently reported on telegram and I've confirmed: Login is usually successful using lightdm (occasionally needs a retry) but fails with gdm even after numerous retries.

As egmde works consistently with both greeters I suspect the Unity8 session configuration, not the Mir support (but the latter is new and should not be discounted entirely).

[gerboland]

As some data points, Unity8 does talk a little with logind to listen for lock/unlock, and request login/logout. Perhaps it is interfering with the new logind support in Mir? Just in case, check out
https://github.com/ubports/unity8/blob/xenial/plugins/Unity/Session/dbusunitysessionservice.h

Also, there is the LightDM integration stuff. If that's still active, it may be getting in the way
https://github.com/ubports/unity8/tree/xenial/plugins/LightDM

Definitely worth watching the DBUS traffic to see if there's something suspicious