Disable -logfile and -modulepath when running with elevated privileges

Could cause privilege elevation and/or arbitrary files overwrite, when
the X server is running with elevated privileges (ie when Xorg is
installed with the setuid bit set and started by a non-root user).

CVE-2018-14665

Issue reported by Narendra Shinde and Red Hat.

Signed-off-by: Matthieu Herrb <matthieu@herrb.eu>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Adam Jackson <ajax@redhat.com>