worker processes: new processes would be spawned while the pool is being
shut down.  Patch by Charles-François Natali.

Eventually I'll actually replace the calls in the tests themselves.

and on OS X with pydebug.

is passed (previously, a select.error with EINVAL would be raised).  Patch
by Charles-François Natali.

Mac OS X and FreeBSD to reduce the chances of a crash instead of a
"maximum recursion depth" RuntimeError exception.
(Patch by Ronald Oussoren)