A few changes for TTL processing:

A few changes for TTL processing:
- Cached messages that reach 0 TTL are considered expired. This prevents
  Unbound itself from issuing replies with TTL 0 and possibly causing a
  thundering herd at the last second. Upstream replies of TTL 0 still
  get the usual pass-through but they are not considered for caching
  from Unbound or any of its caching modules.
- 'serve-expired-reply-ttl' is changed and is now capped by the original
  TTL value of the record to try and make some sense when replying
  with expired records.
- TTL decoding was updated to adhere to RFC8767 section 4 where a set
  high-order bit means the value is positive instead of 0.