Ensure delegations inside mirror zones are properly handled for non-recursive queries

When a resolver is a regular slave (i.e. not a mirror) for some zone,
non-recursive queries for names below that slaved zone will return a
delegation sourced from it.  This behavior is suboptimal for mirror
zones as their contents should rather be treated as validated, cached
DNS responses.  Modify query_delegation() and query_zone_delegation() to
permit clients allowed cache access to check its contents for a better
answer when responding to non-recursive queries.

bin/tests/system/mirror/ns2/sub.example.db.in

@@ -0,0 +1,13 @@
+; 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 3600
+@		SOA	ns2.example. hostmaster 1 3600 1200 604800 3600
+@		NS	ns2.example.
+foo		A	127.0.0.1