daemon: let the xserver log to the journal

If systemd is running, don't re-route the xserver's stdout/stderr to a log
file. Instead hook it up to a journal stream and let the xserver log to the
journal.

gdm thus starts the server with:
  Xorg <other options> -logfile /dev/null -verbose 3

where 3 is the default Xorg log verbosity.

Minor changes by Ray Strode.

https://bugzilla.gnome.org/show_bug.cgi?id=722889