Clean up winsock usage.

This keeps differences which can be cleanly abstracted away clean
 (closesocket, ioctlsocket, getsockopt, setsockopt), and makes
 differences which cannot be cleanly abstracted explicit (SOCKET,
 INVALID_SOCKET, WSAGetLastError/WSASetLastError).
It also gets rid of wsockwrapper.[ch], since it contained just a
 single function.

This can successfully pass the seeking_example tests on
 big.chained.blob over https when built with i686-w64-mingw32 and
 run under wine.
It does not solve the certificate distribution problems with using
 OpenSSL on a real Windows system.