rec: Store additional records as non-auth, even on AA=1 answers

rec: Store additional records as non-auth, even on AA=1 answers

We used to store additional records in AA=1 answers as auth. In addition
to being wrong, it also broke DNSSEC validation if the record was stored
as Indeterminate because while we take care of not validating additional
records when processing an answer, we have no way of knowing in which
section a record was originally located when we retrieve it from the cache.
When an answer becomes too big to fit in the requester UDP payload,
rfc4035 allows the sender to keep records in the additional section
while omitting the corresponding RRSIGs, without setting the TC bit.