sysutils/pam_xdg: add new port

Complete XDG Base Directories support via PAM, including injection of
according environment variables into user sessions, as well as creation
support for per-user $XDG_RUNTIME_DIR.

https://www.sdaoden.eu/code.html#s-toolbox
(cherry picked from commit d255158645dae682063e973978f2f6843b9c4dd4)