[security] warn about existence of config directory on main page

main.php

@@ -302,6 +302,14 @@ if (!empty($_SESSION['auto_blowfish_secret']) &&
     trigger_error($strSecretRequired, E_USER_WARNING);
 }
 
+/**
+ * Check for existence of config directory which should not exist in
+ * production environment.
+ */
+if (file_exists('./config')) {
+    trigger_error($strConfigDirectoryWarning, E_USER_WARNING);
+}
+
 /**
  * Check whether relations are supported.
  */