Generalize the clone function slightly.

* Allow clone where both the child function and stack are null. It's
obviously wrong to ask to call a function without a stack, but it's not
necessarily wrong to supply no stack if you're also not supplying a
function.

* Reimplement fork in terms of the clone function, rather than using the
clone system call directly.

This is intended as a step towards enabling use of pid namespaces.

Change-Id: I03c89bd1dc540d8b4ed1c8fdf6644290744b9e91