unix: link a socket to its peer

We are going to add callback-s for dumping external sockets.
All external sockets are added into unix_list, but for dumping we need
to know all peers.

And one more thing is that a socket is not closed before its peer is
not be dumped. We are going to transfer the socket decriptor in the
callback.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>