Back-port GL #3386 (Do not keep stale NXDOMAIN answers in cache) to BIND 9.18

We did this in BIND 9.19, and whilst I think it a bit too much of a change for 9.16, I would really like to see #3386 (closed) in BIND 9.18 before or as it becomes an ESV. This should have a significant impact on excessive cache bloat in environments with 'stale-cache-enable yes'

No one specific customer, but we encounter cache bloat a lot, and oft-times the bulk is negative content for various reasons. Adding generic stale-ttl to max-ncache-ttl negates the advantages of setting a cap on the TTL of negative content.

We discussed this in one of the engineering meeting and thought it would be a good idea - but I realise now, it never got actioned ...