Make mDNS server response more compatible with RFC 6762 spec.

- dns.Msg construction is now explicit, with comments pointing to the RFC about
  the values of each field.
- dns.Msg responses no longer contain a Question section, which is not allowed
  according to the spec.
- handleQuestion's interface returns both unicast and multicast answer records.
  (Note: These are not yet treated differently by the sendResponse method.)
- removed unused error return value of handleQuestion