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

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

lib/connections/kcp_misc.go

@@ -180,3 +180,10 @@ func (w *sessionClosingStream) Close() error {
 	}
 	return err2
 }
+
+func boolInt(b bool) int {
+	if b {
+		return 1
+	}
+	return 0
+}