Do not attempt to call child_setup on Windows

Do not attempt to call child_setup on Windows

child_setup() is defined to be called after fork() and before exec(),
but Windows' process model does not have fork(): the equivalent of
those two operations is a single CreateProcess() call. This means
that there is no point at which we could call child_setup() and
have it affect only the child's process-global state. At the point
where it is currently executed, it affects the parent's process-global
state instead, which would be actively harmful if we used any
child_setup() function that was not a no-op on Windows.

The equivalent function in GLib, g_spawn_async_with_pipes(), documents
child_setup() as unused on Windows. Do the same here.

In practice, our only use of child_setup() outside tests
is #ifdef DBUS_UNIX anyway, so this change has no practical effect
right now.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=85857
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>