Handle a failed disconnect correctly

If the sender disconnects its socket before sending the disconnect
message, the destination should restart the migration process.  This
patch makes sure that happens.

src/client.h

@@ -30,6 +30,9 @@ struct client {
 
 	/* Have we seen a REQUEST_ENTRUST message? */
 	int     entrusted;
+
+	/* Have we seen a REQUEST_DISCONNECT message? */
+	int     disconnect;
 };