configuration part that is used by all display managers. This is
needed in order to progress towards 'xdm' not acting as wrapper
around DMs, but only be used as Dislplay Manager when XDM is
supposed to be used, or use the real systemd services of the
various DMs (e.g gdm.service, sddm.service).
Part of https://en.opensuse.org/openSUSE:DisplayManagerRework
GDM for example can allready fully be used without XDM (and xorg-x11-server) installed
The sysconfig split out is needed for the GDM Autoconfiguration, which does not rely
on XDM being the wrapper - but having that part common for all DMs is a good thing
OBS-URL: https://build.opensuse.org/package/show/X11:XOrg/xdm?expand=0&rev=279
