be able to send questions with a mDNS unicast response bit set.

see http://tools.ietf.org/html/rfc6762#section-5.4

ex:
```
var question = {
    type  : dns.consts.NAME_TO_QTYPE['A'],
    class : dns.consts.NAME_TO_QCLASS.IN_UNICASTMDNS,
    name  : name + ".local"
};
```