dvdnav: add Demux submodule

It allows to use dvdnav via a VLC access using new dvdnav stream callbacks.

When using a VLC access, dvdnav readahead flag is disabled in order to read
small chunk of data from pf_demux (that is 2kB).

This is disabled by default and must be forced
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>