[feature/request-class] Use the request class in the installer & updater.

Just like common.php database_update.php and install/index.php need to
include the request class files and create an instance for use in
request_var.

PHPBB3-9716

phpBB/install/index.php

@@ -171,6 +171,10 @@ require($phpbb_root_path . 'includes/functions_install.' . $phpEx);
 
 $class_loader = new phpbb_class_loader($phpbb_root_path, '.' . $phpEx);
 $class_loader->register();
+$request = new phpbb_request();
+
+// make sure request_var uses this request instance
+request_var($request, 0); // "dependency injection" for a function
 
 // Try and load an appropriate language if required
 $language = basename(request_var('language', ''));
@@ -813,4 +817,4 @@ class module
 	}
 }
 
-?>
+?>