sysutils/pam_xdg: temporarily hide on -CURRENT due to POLA violation

https://cgit.freebsd.org/src/commit/?id=6e69612d5df1
https://cgit.freebsd.org/src/commit/?id=6b9e6f12e5bb

From a brief look:

• by default base uses /var/run/xdg/<user> while the port uses /var/run/user/<uid>
• session tracking is unconditional in base but opt-in in the port
• base is limited to XDG_RUNTIME_DIR while the port supports other dirs per XDG Base Directory Specification

Migrating from the port requires user intervention:

• adjust/remount /etc/fstab if /var/run/user was under tmpfs(5)
• adjust /etc/pam.d/* to remove unused/unsupported options
• re-login to inherit the new XDG_RUNTIME_DIR value
• restart *user* services that survived re-login