Fix a couple of bugs

3764afc: 	Join mDNS multicast group to receive broadcasted messages
Apparently newer Apple TVs do not send a unicast response to the querying host. They broadcast a message. Host need to join the multicast group to receive such messages.

27d14af: 	Check only the 2 MSBs to detect label format
This fixes what I believe is a misreading of the spec. Both in DNS question and answer name field, only the first two bits of the QNAME / NAME field indicate the label format. Not the entire first byte. See http://www.zytrax.com/books/dns/ch15/#qname.