pam: Always use pam_xdg for session and xdm

This ensure that the XDG_RUNTIME_DIR is always created and the environment
variable is present which is a must for any wayland user (at least).
The only "cost" of it is a directory is created and unused.

Reviewed by: des
Differential Revision: https://reviews.freebsd.org/D44012
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 6b9e6f12e5bbfe347dba8ff8e1e161f9180a1edd)