manager: Don't reuse the same X server when we have a display server

GNOME is going to need a mode of operation in GDM where it manages its
own display.  In this mode of operation, we won't morph the login
screen into a user session by reusing the same X server.  Instead, we
reset the login screen to prepare it for a future login, and then let
GNOME activate its session on its own.

This commit adds the prerequisite work needed to gdm-manager.c, but
only as dead code, since the way to "turn it on" is stubbed out to
always return FALSE.

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