I still don't like the _after_fork() implementation.  Its O(n) where n
== number of threads the parent process had.

Very wasteful when the fork() was done in the most common case of being
followed by an exec and calling os._exit().  It won't scale nicely with
system load (forks will start taking longer and longer the more threads
exist).

Could os.fork() be extended to have an optional will_exec_or_die
parameter that determines if _after_fork() is even called at all? 
Things like subprocess should pass in True.  The default should be False
for compatiblity.