Add a LIST_HEADER macro

To save us from specifying the `next' and `prev' pointers all the time.

It's not perfect, just a tiny bit better.

src/zyklonb.c

@@ -52,9 +52,7 @@ static struct config_item g_config_table[] =
 
 struct plugin_data
 {
-	struct plugin_data *next;           ///< The next link in a chain
-	struct plugin_data *prev;           ///< The previous link in a chain
-
+	LIST_HEADER (plugin_data)
 	struct bot_context *ctx;            ///< Parent context
 
 	pid_t pid;                          ///< PID of the plugin process