ksigfillset is the same for all architectures and
here is no reasons to duplicate the same code many times.

In addition, this patch addes ksigemptyset and ksigaddset.
They will be used in the next patch.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>