gossipd: handle onion errors internally.

As a general rule, lightningd shouldn't parse user packets.  We move the
parsing into gossipd, and have it respond only to permanent failures.

Note that we should *not* unconditionally remove a channel on
WIRE_INVALID_ONION_HMAC, as this can be triggered (and we do!) by
feeding sendpay a route with an incorrect pubkey.

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

gossipd/routing.h

@@ -271,6 +271,7 @@ struct route_hop *get_route(const tal_t *ctx, struct routing_state *rstate,
 void routing_failure(struct routing_state *rstate,
 		     const struct pubkey *erring_node,
 		     const struct short_channel_id *erring_channel,
+		     int erring_direction,
 		     enum onion_type failcode,
 		     const u8 *channel_update);