routing: stricter maxHTLC checks

We require channel updates to have the max HTLC message flag set.

Several flows need to pass that check before channel updates are
forwarded to peers:
* after channel funding: `addToRouterGraph`
* after receiving channel updates from a peer:
  `ProcessRemoteAnnouncement`
* after we update channel policies: `PropagateChanPolicyUpdate`

netann/channel_update_test.go

@@ -182,8 +182,8 @@ func TestUpdateDisableFlag(t *testing.T) {
 
 			// Finally, validate the signature using the router's
 			// verification logic.
-			err = routing.ValidateChannelUpdateAnn(
-				pubKey, 0, newUpdate,
+			err = routing.VerifyChannelUpdateSignature(
+				newUpdate, pubKey,
 			)
 			if err != nil {
 				t.Fatalf("channel update failed to "+