Add unavailable options to hidden options category

Options that are not available (but known in the source code) will
cause an error if they are specified.
Make these options "available" by adding them to the hidden options
category to prevent conf files from failing when shared between binaries
that have different options available.

src/util.h

@@ -263,6 +263,11 @@ public:
      */
     void AddArg(const std::string& name, const std::string& help, const bool debug_only, const OptionsCategory& cat);
 
+    /**
+     * Add many hidden arguments
+     */
+    void AddHiddenArgs(const std::vector<std::string>& args);
+
     /**
      * Clear available arguments
      */