BUG/MINOR: resolvers: relax size checks in authority record parsing

BUG/MINOR: resolvers: relax size checks in authority record parsing

Both boundary checks in the authority record parsing loop of
resolv_validate_dns_response() use >= bufend where they should use
> bufend, causing valid DNS responses with exactly enough bytes to be
rejected as invalid.

The first one, "reader + offset + 10 >= bufend" is too strict since it
prevents 10-byte responses from being accepted as valid while they
are. The second one, "reader + len >= bufend" has the same issue, when
exactly len bytes remain, the check rejects it even though dns_max_name()
already validated it. It may be backported though it is unlikely to ever
be noticed.