[WIP] SOC_U: Implemented some socket functions

This allows FTPony to work.
It's using Berkeley sockets directly since that's what the soc:U service exposes to applications.

Still WIP.
To be consistent with the rest fo Citra, shouldn't this be `socket`? Furthermore, do we know that 3dbrew is actually correct on the lowercase naming here? (e.g. would `Socket` be more appropriate?) I don't see the point of the `SOCU_` prefix as this is already in the `SOC_U` namespace.