Instead of the whole stack.

Stack creation in virtual_alloc_thread_stack and RtlCreateUserStack, as
well as Windows rounding logic, enforce a minimum reserved size of 1MB.

There's a few guard and canary pages used on the bottom, plus a few more
on the top for initial context, and we have to keep a bit of space if we
ever need a stack while doing this, so we clear a bit less than 1MB to
be safe.
Signed-off-by: Rémi Bernon <rbernon@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>