Verify mirror zone AXFRs

Update axfr_commit() so that all incoming versions of a mirror zone
transferred using AXFR are verified before being used.  If zone
verification fails, discard the received version of the zone, wait until
the next refresh and retry.

bin/tests/system/mirror/ns2/named.conf.in

@@ -9,6 +9,15 @@
  * information regarding copyright ownership.
  */
 
+key rndc_key {
+	secret "1234abcd8765";
+	algorithm hmac-sha256;
+};
+
+controls {
+	inet 10.53.0.2 port @CONTROLPORT@ allow { any; } keys { rndc_key; };
+};
+
 options {
 	query-source address 10.53.0.2;
 	notify-source 10.53.0.2;
@@ -19,3 +28,18 @@ options {
 	listen-on-v6 { none; };
 	recursion no;
 };
+
+zone "verify-axfr" {
+	type master;
+	file "verify-axfr.db.signed";
+};
+
+zone "verify-unsigned" {
+	type master;
+	file "verify.db.in";
+};
+
+zone "verify-untrusted" {
+	type master;
+	file "verify-untrusted.db.signed";
+};