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/example.db.in

@@ -12,3 +12,4 @@ $TTL 3600
 @		NS	ns2
 ns2		A	10.53.0.2
 foo		A	127.0.0.1
+sub		NS	ns2