* clean up some code, add "mp3" short cut.

 * clean up some code, add "mp3" short cut.
   And now if this demux has been forced it will (try) to play the whole
file even if the stream is heavily corrupted (like PS demux).
   But now, it makes less assumptions on the stream, so report any
problem if it mis-recognizes others files as mpeg audio (specially PS file).