Sorry, I discovered that the reaper wasn't working properly, and that
slightly more radical changes were necessary.  I've attached a new
version of the patch set.  The only patch that changed here is the last
one, but I include all of them again for simplicity.

I deleted the old 'wip-thread-safe-popen' branch and pushed a new one
called 'thread-safe-popen', because I believe this one finally does the
entire job correctly.

      Mark