unix: Add fake queuer for standalone stream sockets in established state

This makes the second end of socketpair to live till post_open.
We need it alive if we want to restore message queue later.
Otherwise, we do not have a queuer, which fd is used to actually
write messages.

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