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.

I do not have access to any Apple TVs running older software. Mine runs version 7. If this PR breaks anything do let me know. I will try to help.