Replace the "mirror" zone option with "type mirror;"

Use a zone's 'type' field instead of the value of its DNS_ZONEOPT_MIRROR
option for checking whether it is a mirror zone.  This makes said zone
option and its associated helper function, dns_zone_mirror(), redundant,
so remove them.  Remove a check specific to mirror zones from
named_zone_reusable() since another check in that function ensures that
changing a zone's type prevents it from being reused during
reconfiguration.

bin/tests/system/mirror/README

@@ -0,0 +1,17 @@
+Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+
+See COPYRIGHT in the source root or http://isc.org/copyright.html for terms.
+
+This test checks whether zones configured with "type mirror;" behave as
+expected.
+
+ns1 is an authoritative-only server.  It only serves the root zone, which is
+mirrored by ns3.
+
+ns2 is an authoritative-only server.  It serves a number of zones, some of which
+are delegated to it by ns1 and used in recursive resolution tests aimed at ns3
+while others are only served so that ns3 has a primary server to mirror zones
+from during various tests of the mirror zone implementation.
+
+ns3 is a recursive resolver.  It has a number of mirror zones configured.  This
+is the only server whose behavior is being examined by this system test.