Introduce dnssec-policy configuration

This commit introduces the initial `dnssec-policy` configuration
statement. It has an initial set of options to deal with signature
and key maintenance.

Add some checks to ensure that dnssec-policy is configured at the
right locations, and that policies referenced to in zone statements
actually exist.

Add some checks that when a user adds the new `dnssec-policy`
configuration, it will no longer contain existing DNSSEC
configuration options.  Specifically: `inline-signing`,
`auto-dnssec`, `dnssec-dnskey-kskonly`, `dnssec-secure-to-insecure`,
`update-check-ksk`, `dnssec-update-mode`, `dnskey-sig-validity`,
and `sig-validity-interval`.

Test a good kasp configuration, and some bad configurations.

bin/tests/system/checkconf/bad-kasp4.conf

@@ -0,0 +1,23 @@
+/*
+ * 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.
+ */
+
+// Bad kasp configuration because this has an invalid duration for
+// signatures-refresh.
+dnssec-policy "badduration" {
+	signatures-refresh PT20Sabcd;
+};
+
+zone "example.net" {
+	type master;
+	file "example.db";
+	dnssec-policy "badduration";
+};
+