Additions to librustuv for low level networking

[mrmonday]

This pull request adds some necessary function definitions and types for doing low level networking with Rust.

[alexcrichton]

In the past we've been growing hesitant about exposing more platform-specific functionality from liblibc as it's already a bit of a mess. The bindings for librustuv seem fine, but perhaps the liblibc changes could be relegated to the relevant library outside of the compiler?

[mrmonday]

So as you might have guessed, this is the beginnings of refactoring the raw socket stuff that I had before in a way that doesn't require massive additions to the standard libraries. I have a follow up pull request which will require some subset of the additions to liblibc; I have a separate library which requires the rest.

Perhaps I could figure out what additions are necessary and change this pull request to only add them? Or would you rather I include the additions with my follow-up pull request and only add the librustuv things here?

[alexcrichton]

The libuv changes for now and possibly more later sounds good to me!

[mrmonday]

I've updated this pull request to just have the librustuv changes - I will submit the liblibc changes alongside the other modifications later on.