logind: fail on CreateSession if already in session

Right now, if you're already in a session and call CreateSession, we
return information about the current session of yours. This is highy
confusing and a nasty hack. Avoid that, and instead return a commonly
known error, so the caller can detect that.

This has the side-effect, that we no longer override XDG_VTNR and XDG_SEAT
in pam_systemd, if you're already in a session. But this sounds like the
right thing to do, anyway.