KDM and LightDM (at least with autologin) call the xsession-script with
two arguments: the first is the path of the xsession script itself,
while the second one are the actual arguments. The line to re-exec the
script under systemd-cat only forwarded a single argument, therefore
breaking LightDM and KDM login. This commit fixes the issue by always
forwarding all the arguments.