daemon: add wrapper for launching X session
    
    This commit adds a utility for launching an X server and session
    together.
    
    This utility works a lot like xinit, but makes use of
    modern X features (like -displayfd) and has GDM specific integration
    (like reading from gdm configuration, and optionally running through
     /etc/gdm/Xsession)
    
    The eventual idea is to get the main GDM code out of the X server launching
    business and instead farm the logic off to an unprivileged helper
    that gets run within the logind session of the user.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=744764