Handle DNAME lookup via itself

Handle DNAME lookup via itself

When answering a query, named should never attempt to add the same RRset
to the ANSWER section more than once.  However, such a situation may
arise when chasing DNAME records: one of the DNAME records placed in the
ANSWER section may turn out to be the final answer to a client query,
but there is no way to know that in advance.  Tweak the relevant INSIST
assertion in query_respond() so that it handles this case properly.
qctx->rdataset is freed later anyway, so there is no need to clean it up
in query_respond().