Service descriptors are a set of file descriptors which isn't
intercepted with restored file descriptors.

In all cases, when we don't need to restore file descriptors, we can
skip all magic of service-fd and just save descriptor numbers in a
static array.
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>