Reduce windows header includes in our own headers.

Reduce windows header includes in our own headers.

It turns out that absolutely everything that was including
windows.h was doing so needlessly; our headers don't need it,
so we should just include winsock2.h (since that's where
struct timeval is defined).

Pre-2.0 code will use the old headers, which include windows.h
for them, so we aren't breaking source compatibility with 1.4.

This solves the bug where we were leaving WIN32_LEAN_AND_MEAN
defined, in roughly the same way that buying an automobile
solves the question of what to give your coachman for boxing
day.