use DNS_DB_NONSEC3 flag when copying non-dnssec records

when copying the non-dnssec records in receive_secure_db(),
use DNS_DB_NONSEC3 so we don't accidentally create nodes in
the main tree for NSEC3 records. this was a long-standing error
in the code, but was harmless in the RBTDB.

lib/dns/zone.c

@@ -17220,7 +17220,7 @@ receive_secure_db(void *arg) {
 		goto failure;
 	}
 
-	result = dns_db_createiterator(rawdb, 0, &dbiterator);
+	result = dns_db_createiterator(rawdb, DNS_DB_NONSEC3, &dbiterator);
 	if (result != ISC_R_SUCCESS) {
 		goto failure;
 	}