unix: Support semi-closed stream connection

Unix socket and have no peer, but be in established state.
Such types of sockets appear when one end of a connection
closes and leaves the peer in full-shutdown state.

Check this and handle properly on restore.

Reported-by: Florian Gross <Florian.S.Gross@web.de>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>