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