gossipd: add flag for locally disabling channel.

We used to just manually set ROUTING_FLAGS_DISABLED, but that means we
then suppressed the real channel_update because we thought it was a
duplicate!

So use a local flag: set it for the channel when the peer disconnects,
and clear it when channeld sends a local update.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

gossipd/routing.h

@@ -55,6 +55,9 @@ struct chan {
 	/* Index in broadcast map, if public (otherwise 0) */
 	u64 channel_announcement_index;
 
+	/* Disabled locally (due to peer disconnect) */
+	bool local_disabled;
+
 	u64 satoshis;
 };