[Libav-user] The proper approach to pass options to nested format

[Zhang Rui]

In http.c, a field named chained_options is used to pass options to
underlying tcp protocol. In hls.c, options (only for http) is copied explicity
by name to pass down to nested url context.

But, when open a m3u8 file in file/pipe protocol, which contains
segments in http url, the "user-agent" option doesn't accepted by
outer file/pipe protocol, nor be passed by any callback.

And same problem exists in concat.c/concatdec.c

I'd like to know, is it OK to introduce something like
int (*read_header2)(struct AVFormatContext *, AVDictionary **options);
in AVInputFormat