unix: Link all sockets in unix_sockets list

List "unix_sockets" is used to link external sockets.
Change this, to link all unix sockets there. To differ
really external sockets in fix_external_unix_sockets(),
we may check for socket's already_dumped value.

This may be useful to traverse over all unix sockets
in a namespace.

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