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
Please name this:
```
exports.lookup = function lookup(hostname, options, callback) {
```