manager: support just-in-time reauthentication for non-GDM sessions

Right now, gnome-shell can't unlock screens running on an X server that
isn't managed by GDM (say Xvnc or startx).  This is because GDM handles
the backend processing for unlocking, and it handles that backend
processing from the worker associated with the session.  If there is no
worker associated with the session (as is the case with Xvnc and startx),
then there's no process to handle reauthentication.

This commit notices that case, and creates a transient worker on the fly
just to perform one off authentication for unlock of non-GDM managed
sessions.

https://bugzilla.gnome.org/show_bug.cgi?id=726283