suppress logging of RPZ passthru

A request from Andrew Fried:

A recurring question I get from RPZ users is, "how can I disable logging rpz passthru's". What's basically happening is that folks create a local rpz whitelist to insure their own infrastructure doesn't get blocked, and since they see a lot of lookups for their own infrastructure they produce a lot of log entries.

It would be nice if there were a way to suppress passthru type log entries.