Support using custom sockets with libuv or libnative

This is akin to supporting custom file descriptors, but for sockets. This allows for the implementation of raw sockets in an external library, as well as integration with other libraries that use sockets such as zeromq.

It exposes a lot more of liblibc than might be desirable, but the other alternative is to add a lot of additional code which probably shouldn't be in the standard library (it quickly explodes into something monstrous like pull #11410, and that was by no means complete). This should be a minimal subset which will allow other libraries to build upon.
It's unfortunate to have this duplicated code between libnative and librustrt, but I'm not entirely certain how to otherwise do it. Likewise, it's a bit unfortunate to have all the new libc definitions as they're already in libnative and we're trying to keep the size of libc under control.