std.c.windows.winsock: Win64 and type fixes

The SOCKET type was wrong and caused 64-bit programs using WinSock functions that passed SOCKETs indirectly (like `select`) to not work.

I noticed the signedness of `socklen_t` was also different from the C headers, so I fixed that as well.

As far as I can tell, this was the only problem preventing std.socket from working on Win64.

The servent fix also fixes std.socket unit tests.