- After boo#968405 got fixed in /etc/rpm/macros.systemd, resurrect the old
behavior of %postun (boo#968405).

- xdm.tar.bz:
* /etc/X11/xdm/sys.xsession:
fix regression from fix for boo#967002 (boo#968172).
* /etc/X11/xdm/Xsession:
+ Add comments for documentation
+ Fix semantics
+ Don't use $HOME/.xinitrc as session script. Let sys.xession
handle this.
+ Make argument parser more robust:
add parse error detection.
* Add an 'xsession' desktop file: this should offer a session
where the $HOME/.xsession is used (boo#968631).

- Remove %service_del_postun display-manager.service
from %postun macro (boo#968405).