Make v0.8.5 actually work on windows.

Next time, when writing code specifically intended to work around
platform misfeatures, it would probably be a good idea to actually
test it on that platform; this revealed a number of bugs.

I'm not certain that all of these win multiprocessing hacks are
even a good idea; they're introducing a bunch of complexity
into the startup code paths and make the API harder to understand,
and I'm not sure if I can really make them correct in all cases,
either :(.