If ipv6 enabled, listen and proxy ipv6 requests as well as ipv4

Uses the presence of an ipv6 loopback network address to indicate that ipv6 is enabled.
Fixes https://github.com/meteor/meteor/issues/1140

This problem was noticed on Windows where the default is for ipv6 to be enabled, and for localhost to resolve to `::1` then `127.0.0.1`.  What happens is that if you enter `localhost:3000` into the Chrome address bar, it does connect. But it will usually have to fall back to long polling rather than using websockets.
It also delays things by (up to) a second - which when you are aware of it is quite noticeable.

Finally, the initial thing that triggered this was that there is what looks like an error on the Javascript console, but is a side effect of the falling back to long polling when the initial websocket is delayed.