manager: Make migrated a special case when starting the session

We have three cases for starting a session:

 1) We migrate to an existing session. Simply tear down state
 and switch.

 2) We're starting a new session, and it's a traditional Xorg
 server. Tear down the greeter, and keep the same X server and
 GdmDisplay, and start a full user session.

 3) We're starting a new session, but it's a display server. Keep the
 greeter around and give it a new seed session, and start a full user
 session.

The last two cases are really the two sides of the same coin, while the
migrated case is really a fast-path special case that doesn't involve
launching a user session in any case, so use a "goto out;" as a fast
return rather than an if/else.

https://bugzilla.gnome.org/show_bug.cgi?id=726380