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.
I believe you have just solved a mysterious bug I've been encountering on Windows when using the `libuv` backend! It doesn't seem to cause issues on other platforms. I'm not sure what the best way to solve this is, other than documenting it or maintaining a list of active sockets.