pam_systemd: do not set XDG_RUNTIME_DIR if the session's original user is not the...

pam_systemd: do not set XDG_RUNTIME_DIR if the session's original user is not the same as the newly logged in one

It's better not to set any XDG_RUNTIME_DIR at all rather than one of a
different user. So let's do this.

This changes the bus call parameters of CreateSession(), but that is
explicitly an internal API hence should be fine. Note however, that a
logind restart (the way the RPM postinst scriptlets do it) is necessary
to make things work again.