[WIP] SOC_U: Implemented some socket functions

This allows ftpony and ftbrony to work.
It's using Berkeley sockets directly since that's what the soc:U service exposes to applications.

Still WIP.
You should indent the preprocessor statements, perhaps like this:

```c++
#if EMU_PLATFORM == PLATFORM_WINDOWS
#   define WSAEAGAIN WSAEWOULDBLOCK
#   define ERRNO(x) WSA##x
#   define GET_ERRNO WSAGetLastError()
#   define poll(x, y, z) WSAPoll(x, y, z);
#else
#   define ERRNO(x) x
#   define GET_ERRNO errno
#endif
```

Kinda hard to read otherwise.