give rc-status packages a pin of -1

give rc-status packages a pin of -1

It would previously return a pin of 0, which is an invalid value, but
the intend is that versions which are only in the dpkg/status file can't
be selected for installation (= can't be a candidate) which is what a
negative pin assures.

This helps with the communication to EDSP solvers as they neither know
about the rc-state (yet) nor that they shouldn't choose this version.
Ideally they shouldn't be told about such versions at all as there is
nothing to be solved here, but we will get there eventually.