MXS-1220: Add alter maxscale MaxAdmin command

A set of the core MaxScale parameters can now be altered at runtime. This
set consists of the authentication timeouts and the admin interface
authentication. Other parameters either can't be modified due to internal
limitations or aren't sensible to modify at runtime.

server/core/maxscale/config.h

@@ -149,4 +149,11 @@ void config_add_module_params_json(const MXS_MODULE* mod,
  */
 void fix_section_name(char *section);
 
+/**
+ * @brief Serialize global options
+ *
+ * @return True if options were serialized successfully
+ */
+bool config_global_serialize();
+
 MXS_END_DECLS