unux: postpone dumping sockets v2

Unix sockets are dumped, when a peer socket is found.
We are going to dump external sockets with help plugins. For the we need
to set the USK_CALLBACK flags in unix entry. Currently a socket is
dumped immediately when it's transfered, but we can be sure that a
socket is not external, only when we have its peer.

v2: add comments in code
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>