pam: Add pam_xdg module

This is a module to setup the XDG directories and environment variables.
For now the only usage is to have a XDG_RUNTIME_DIR environment setup at
user login.
All other environment variable have a default fallback so no need to export
them in this module.
The directory is created according to the XDG Base directory specification.

The default base directory is /var/run/xdg/<username> but can be configured
using the runtime_dir=<dir> module option.

According to the spec the directory *must* not survive a reboot so adding
var_run_enable="YES" to rc.conf is highly recommanded.

Reviewed by: des, pauamma (manpages)
Differential Revision: https://reviews.freebsd.org/D44011
Sponsored by: Beckhoff Automation GmbH & Co. KG