Thread support is now required

Long ago we supported systems where there was no support for
threads, or where the threading library was broken. We shouldn't
have do that any more: on every OS that matters, threads exist, and
the OS supports running threads across multiple CPUs.

This resolves tickets 9495 and 12439.  It's a prerequisite to making
our workqueue code work better, since sensible workqueue
implementations don't split across multiple processes.