cmdutils/avtools: simplify show_help() by using av_opt_child_class_next()

cmdutils.h

@@ -154,6 +154,12 @@ typedef struct {
 
 void show_help_options(const OptionDef *options, const char *msg, int mask, int value);
 
+/**
+ * Show help for all options with given flags in class and all its
+ * children.
+ */
+void show_help_children(const AVClass *class, int flags);
+
 /**
  * Parse the command line arguments.
  *