Change listen() in DnsServer to use socket creation callback interface.

Per reference in
[10.26 UDP docs](http://nodejs.org/api/dgram.html#dgram_udp_datagram_sockets):

> Important note: the behavior of dgram.Socket#bind() has changed in v0.10 and is always asynchronous now.

The listen interface calls bind() on the underlying dns_server/Socket, and immediately invokes its own callback if provided. This has been changed to call it from inside the callback arg in the new `bind(port, [adr], [cb])` format, which fixes test breakage in `test_dns_server.coffee` due to trying to call `address` too soon.