Fix find() with an explicit constraint name (... { key => $cname } )

Fix find() with an explicit constraint name (... { key => $cname } )

Fix the code being overly lax on evaluation of the combined
condition (user supplied + $rs inherited). If we can not prove
that a constraint is deteministic - we throw

Also fix the heuristics invoked without a key attr. to not
consider constraints for which NULL values have been supplied