but if the child thread calls exit(-1) from fatal, the frisbee process
exits, but with a code of zero; i.e., the child exit code is lost.
Granted, a multi-threaded program should not be calling exit willy-nilly,
but it does so we deal with it as follows.

Since the child should never exit during normal operation (we always
kill it), if it does exit we know there is a problem.  So, we catch
all exits and if it is the child, we set a flag.  The parent thread
will see this and exit with an error.

Since I don't understand this fully, I am making it a FreeBSD-only
thing for now.