Transcode channel naming in help output

Uses AOUT_CHANS_ channel listing in transcode. They were same, but makes it easier to read and extend in future, if needed.

Shows following info on channel-amount info with 'vlc -p transcode'

--sout-transcode-channels {0 (default), 1 (mono), 2 (stereo), 3 (2.1), 4 (4.0), 5 (5.0), 6 (5.1), 7 (7.0), 8 (7.1), 9 (8.1)}

Makes more clear what channel amount provides what channel-layout