lib/config, lib/connections: Configurables for KCP, disable by default

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4030

lib/config/wrapper.go

@@ -391,6 +391,9 @@ func (w *Wrapper) ListenAddresses() []string {
 		switch addr {
 		case "default":
 			addresses = append(addresses, DefaultListenAddresses...)
+			if w.cfg.Options.DefaultKCPEnabled { // temporary feature flag
+				addresses = append(addresses, DefaultKCPListenAddress)
+			}
 		default:
 			addresses = append(addresses, addr)
 		}