implement searching of geoip2 database

- revise mapping of search terms to database types to match the
  GeoIP2 schemas.
- open GeoIP2 databases when starting up; close when shutting down.
- clarify the logged error message when an unknown database type
  is configured.
- add new geoip ACL subtypes to support searching for continent in
  country databases.
- map geoip ACL subtypes to specific MMDB database queries.
- perform MMDB lookups based on subtype, saving state between
  queries so repeated lookups for the same address aren't necessary.

bin/named/include/named/geoip.h

@@ -9,18 +9,15 @@
  * information regarding copyright ownership.
  */
 
-#ifndef _GEOIP_H
-#define _GEOIP_H
+#pragma once
 
-#ifdef HAVE_GEOIP
-#include <GeoIP.h>
-#include <GeoIPCity.h>
-#endif /* HAVE_GEOIP */
-
-void named_geoip_init(void);
-void named_geoip_load(char *dir);
-
-#ifdef HAVE_GEOIP
 extern dns_geoip_databases_t *named_g_geoip;
-#endif /* HAVE_GEOIP */
-#endif
+
+void
+named_geoip_init(void);
+
+void
+named_geoip_load(char *dir);
+
+void
+named_geoip_shutdown(void);