logind: automatically determine client side PID if GetSessionByPID() is called with a PID == 0
bus: add sd_bus_get_current() bus call to determine message that is currently being dispatched
update TODO
logind: add virtual object paths that always can be used to refer to the callers session, user, seat or machine object

This way clients can skip invoking GetSessionByPID() for their own PID
or a similar call to access these objects.
update TODO
bus: never report nodes as children that do not have the right prefix