Re: pg_threads.h take II

I decided to split the API patch into two parts, to highlight
something interesting:

v3-0002 pg_threads.h with support for <pthread.h> or <threads.h>
v3-0003 adds <windows.h> option for native Windows API

If we are ready to make <threads.h> a hard requirement for Visual
Studio builds, and willing to use the <pthread.h> implementation that
ships with MinGW, then we don't actually need v3-0003. That means
that we're left with almost no code, mostly just renaming wrappers and
comments.

CI passes with or without v3-0003.

I also fixed the commit message for v3-0006 which I'd failed to update
(I abandoned my thread-exit invention and just went with tss_t in the
end). No other changes from v2.