windows: don't use uppercase in include filename

Change <Winsock2.h> to <winsock2.h>. The former breaks
cross-compilation with mingw.
stream: add an API to make streams do blocking writes

This patch adds the `uv_stream_set_blocking` API which makes all
uv_write calls to that stream blocking. It currently only works for
pipes, on windows.
windows: use WSAGetLastError(), not errno

setsockopt() doesn't touch errno on failure. Use WSAGetLastError()
instead.