Give external_networks unique key a name.

At least our schemadiff script assumes that unique keys have names.
mysqldump generates them as well.

sql/database-create.sql

@@ -1290,7 +1290,7 @@ CREATE TABLE `external_networks` (
   `min_vlan` int(11) NOT NULL default '256',
   `max_vlan` int(11) NOT NULL default '1000',
   PRIMARY KEY  (`network_id`),
-  UNIQUE KEY  (`node_id`)
+  UNIQUE KEY `node_id` (`node_id`)
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1;
   
 --

sql/updates/4/234

@@ -16,7 +16,7 @@ sub DoUpdate($$$)
 		     " `min_vlan` int(11) NOT NULL default '256', ".
 		     " `max_vlan` int(11) NOT NULL default '1000', ".
 		     " PRIMARY KEY  (`network_id`), ".
-		     " UNIQUE KEY  (`node_id`) ".
+		     " UNIQUE KEY `node_id` (`node_id`) ".
 		     ") ENGINE=MyISAM DEFAULT CHARSET=latin1");
     }