unix: Rework peer transfer in open_unixsk_pair_master()

After previous patch, master and slave ends of socketpair
are owned by the only task. So, we may avoid using
of send_desc_to_peer() of the second end, and just
reopen it with right pid.

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