Apply the same logic used for subtitle stream selection to audio and video
stream selection.

Make the counting starts from 0, and disable the corresponding playbck
when the selected stream number is negative.

See the thread:
" [FFmpeg-devel] [PATCH] Add documentation for -ast, -vst, -sst".

Originally committed as revision 17693 to svn://svn.ffmpeg.org/ffmpeg/trunk
