zdtm: Send two descriptors in two SCMs

Only the send code is altered, as upon receiving kernel
merges all scm_rights int one. CRIU relies on this merge
and this is to catch situations if the kernel suddenly
stops doing this.

Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>