Rename _fileno() to fileno() and add for Windows

Note that the tests fail because this needs an updated libuv. I couldn't bump the version here because if I do some FS tests fail. @saghul, I think this is what you referred to when you were saying you need to finish the fs refactor before bumping the version?

Manually bumping the version and testing this on Windows and Linux works OK.
It's a HANDLE on Windows.