bug 9 -- sibling threads not infinite looping

[Neil Jerram]

Is this still a problem?  I can't reproduce it with either stable or
unstable.

For me,

- `guile -s bug9.scm' outputs about 15 `a's and `b's, and then stops,
  as the main thread has exited

- `guile -l bug9.scm' continues outputting `a's and `b's forever, and
  also performs normal REPL interaction at the same time.

In both cases, this is what I would expect.

[I've found a new problem, though - interrupting in the `guile -l'
case causes a SEGV.]

        Neil