Support for NSEC3 in dnssec-policy

Implement support for NSEC3 in dnssec-policy.  Store the configuration
in kasp objects. When configuring a zone, call 'dns_zone_setnsec3param'
to queue an nsec3param event. This will ensure that any previous
chains will be removed and a chain according to the dnssec-policy is
created.

Add tests for dnssec-policy zones that uses the new 'nsec3param'
option, as well as changing to new values, changing to NSEC, and
changing from NSEC.

bin/tests/system/nsec3/ns3/template.db.in

@@ -0,0 +1,25 @@
+; 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.
+
+$TTL 300
+@		IN	SOA  mname1. . (
+			1       ; serial
+			20      ; refresh (20 seconds)
+			20      ; retry (20 seconds)
+			1814400 ; expire (3 weeks)
+			3600    ; minimum (1 hour)
+			)
+
+			NS	ns3
+ns3			A	10.53.0.3
+
+a			A	10.0.0.1
+b			A	10.0.0.2
+c			A	10.0.0.3
+