Review request for 5049299

[Andrew Haley]

Martin Buchholz wrote:

> I've been thinking we should go further than having
> compile-time support for choosing the fork-method
> and add a system property to select among the
> 4 different ways of starting a subprocess.
> The kinds of changes being made will still be risky
> and it would be nice to have a way to revert to the
> safer fork() strategy.

That's very sensible.

Andrew.