resolved: implement query coalescing

resolved: implement query coalescing

Implement dns_transaction_make_packet_mdns(), a special version of
dns_transaction_make_packet() for mDNS which differs in many ways:

a) We coalesce queries of currently active transaction on the scope.
   This is possible because mDNS actually allows many questions in a
   to be sent in a single packet and it takes some burden from the
   network.

b) Both A and AAAA query keys are broadcast on both IPv4 and IPv6
   scopes, because other hosts might only respond on one of their
   addresses but resolve both types.

c) We discard previously sent packages (t->sent) so we can start over
   and coalesce pending transactions again.