files: Allow to send unix sockets over unix sockets

Everything is ready. Message queue restores are in
the second stage of open for all types of unix sockets.
We just need to make scm wait before restore_unix_queue()
and allow to dump such scm context.

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