unix: Handle service socket on dump and restore

Service connection is actually an 'external' one from unix sockets engine POV,
but we don't want to dump it as such. Thus, we explicitly find one and dump it
as half-closed connection. On restore we push an artificial message into it
to report to the program that the dump-request was served, but the program is
restored.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>