In addition to linking with Winsock 2, also use winsock2.h.
This allows to turn on IPv6 support on by default under MSW, so turn it
on under Unix too for consistency.
Predefine _WINSOCK_DEPRECATED_NO_WARNINGS to avoid warnings about
deprecated (but still available and very unlikely to be ever removed)
Winsock 1 functions that we still use.