Cygwin: fork: Remember child not before success.

Cygwin: fork: Remember child not before success.

Do not remember the child before it was successfully initialized, or we
would need more sophisticated cleanup on child initialization failure,
like cleaning up the process table and suppressing SIGCHILD delivery
with multiple threads ("waitproc") involved.  Compared to that, the
potential slowdown due to an extra yield () call should be negligible.