multi: add option to disable route blinding, rejecting at link

Add an option to disable route blinding, failing back any HTLC with
a blinding point set when we haven't got the feature enabled.

Note that this commit only handles the case where we're chosen as the
relaying node (where the blinding point is in update_add_htlc), we'll
add handling for the introduction node case once we get to handling of
blinded payloads).

peer/brontide.go

@@ -365,6 +365,11 @@ type Config struct {
 	// this across multiple Peer struct instances.
 	PongBuf []byte
 
+	// Adds the option to disable forwarding payments in blinded routes
+	// by failing back any blinding-related payloads as if they were
+	// invalid.
+	DisallowRouteBlinding bool
+
 	// Quit is the server's quit channel. If this is closed, we halt operation.
 	Quit chan struct{}
 }
@@ -1155,6 +1160,7 @@ func (p *Brontide) addLink(chanPoint *wire.OutPoint,
 		HtlcNotifier:            p.cfg.HtlcNotifier,
 		GetAliases:              p.cfg.GetAliases,
 		PreviouslySentShutdown:  shutdownMsg,
+		DisallowRouteBlinding:   p.cfg.DisallowRouteBlinding,
 	}
 
 	// Before adding our new link, purge the switch of any pending or live