add syntax and setter/getter functions to configure max-ixfr-ratio

bin/tests/system/checkconf/warn-maxratio1.conf

@@ -0,0 +1,17 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+zone example {
+	type master;
+	masterfile-format map;
+	file "example.db";
+	max-ixfr-ratio 101%;
+};