Made Pybuild list variables inherit from their parents in the class heirarchy

To prepare the way for #221 (closed) (eventually, and hopefully).

I'm slightly hesitant to make this change, since it's a nontrivial API change, departs from Portage's Package Manager Specification, and #221 (closed) may not even ever be implemented (e.g. if a better method is found). Plus after looking further into #221 (closed), I think it wouldn't be too difficult to support init fully.