daemon: run greeter in its own distinct session

Right now before launching the greeter, we create this
little ad hoc session that only runs through some of the
OS machinery for session registration.

This means not only is there duplicate code with GDMs
real session handling functions, but the greeter is
potentially running in an incomplete session.

This commit, inspired by the work of Lennart and Kay,
changes GDM to run its greeter session through a session
worker just like any other session.