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.

lib/dns/zt.c

@@ -181,7 +181,8 @@ dns_zt_find(dns_zt_t *zt, const dns_name_t *name, unsigned int options,
 		 * instead of returning a SERVFAIL.
 		 */
 		if ((options & DNS_ZTFIND_MIRROR) != 0 &&
-		    dns_zone_ismirror(dummy) && !dns_zone_isloaded(dummy))
+		    dns_zone_gettype(dummy) == dns_zone_mirror &&
+		    !dns_zone_isloaded(dummy))
 		{
 			result = ISC_R_NOTFOUND;
 		} else {