server: Use a separate function and flag to communicate that the initial status of an async is not known yet.

Mostly just to simplify the interface, so that we don't need to use the return
value to communicate this.
Signed-off-by: Zebediah Figura <zfigura@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>