gdm: always run gdm on VT1
    
    This drops automatic VT allocation schemes for the initial display in
    favour of a compile time hardcoded VT assignment. The automatic
    allocation schemes are inherently racy since a simpler output to the
    console might already influence it and result in gdm running on another
    than the intended VT.
    
    This patch adds a --with-initial-vt= switch to configure which may be
    used to set the VT gdm will run the initial server on. It defaults to 1.
    
    https://fedoraproject.org/wiki/Features/DisplayManagerRework
    
    Backported by Ray Strode
    
    https://bugzilla.gnome.org/show_bug.cgi?id=511168