This sets up a more correct session as there are key tasks that
need to be performed by PAM. E.g. systemd will allocate cgroups
and start base services.
In order to easily handle this as a system service the mapping of
displays is now done via a configuration file.
It is the most common init system these days so it should not be
hidden in the contrib/ directory.
This also removes all old SysV files from the contrib packages.