Win32 signals patch

Here's the latest win32 signals code, this time in the form of a patch
against the latest shapshot. It also includes the replacement of kill()
with pqkill() and sigsetmask() with pqsigsetmask().

Passes all tests fine on my linux machine once applied. Still doesn't
link completely on Win32 - there are a few things still required. But
much closer than before.

At Bruce's request, I'm goint to write up a README file about the method
of signals delivery chosen and why the others were rejected (basically a
summary of the mailinglist discussions). I'll finish that up once/if the
patch is accepted.

Assuming it passes review, this code is ready to be applied to HEAD.

//Magnus