[MLton] Finished (?) MLton.Child

[Henry Cejtin]

I meant to mention previously that with regards to the `race' condition on
searching for the right element of the path, under Unix there is no race:
you just fork and then the child tries to exec each element of the path.
If one succeeds then you never continue with the loop.  If you fall off the
end then you failed.  (One problem is that you are now in the child, but that
is certainly the way that execvp() in the C library work.)