resolved: put size limit in DnsAnswer size to UINT16_MAX

resolved: put size limit in DnsAnswer size to UINT16_MAX

The three answer sections can only carry up to UINT16_MAX entries, hence
put a hard upper limit on how far DnsAnswer can grow. The three count
fields in the DNS packet header are 16 bit only, hence the limit.

If code actually tries to add more than 64K RRs it will get ENOSPC with
this new checking.

And similar to DnsQuestion.