net: do not prefer IPv4 addresses during resolution.

Currently, the address resolution family defaults to IPv4. This commit removes that preference, and instead attempts to resolve to a family suitable for the host.

@tjfontaine is this along the lines of what you wanted? This is my first time venturing into the core C++ layer, so forgive me if it's way off.

Closes #7637
The intent behind the `require('dns')` was, as I understand it, to not incur the penalty of loading `dns` if it won't be used (for example, if you're requiring net for `net.createServer`).