cmdutils: add opt_default2().

It stores options in a dictionary to be passed to new open calls.

It will replace opt_default once all the pieces are in place.

cmdutils.h

@@ -43,6 +43,7 @@ extern const char **opt_names;
 extern AVCodecContext *avcodec_opts[AVMEDIA_TYPE_NB];
 extern AVFormatContext *avformat_opts;
 extern struct SwsContext *sws_opts;
+extern AVDictionary *format_opts, *video_opts, *audio_opts, *sub_opts;
 
 /**
  * Initialize the cmdutils option system, in particular