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.