channel: Disable channels that are lost via a channel_update

Before exiting, `channeld` constructs and sends a `channel_update`
marking the channel as disabled. This is the pro-active signalling
that the channel may no longer be used.

daemon/routing.h

@@ -7,6 +7,7 @@
 #include <ccan/htable/htable_type.h>
 
 #define ROUTING_MAX_HOPS 20
+#define ROUTING_FLAGS_DISABLED 2
 
 struct node_connection {
 	struct node *src, *dst;