It turns out that at least on Windows, spurious unparks are *not*
possible, and in fact triggering them breaks some RTL synchronization
primitives. For instance, if you have a pending unpark going into a
contended `RtlEnterCriticalSection` call, it will never unblock. In
other words, the Windows API worked exactly how I thought it did, and
it's only the NetBSD/Illumos one which is dumb. This is actually exactly
why Windows 8 introduced the parking API despite alertable sleeps being
a thing!

This commit doesn't yet deal with making NetBSD work, nor does it even
compile I imagine. The next commit will fix everything back up.

This reverts commit c518593e9793a2aed4e0173348aff2cbef58b717.