update the heuristic: try to keep `current_song', not play_off

keep current song in a dedicated variable playlist.songs[play_off] is not a good representation for the current playing song after all. After a `load' command, or a flush request, the play_off changes but we're still playing the "old" song. This saves the current song in a dedicated variable that's updated upon playlist_advance and playlist_previous, thus givin a more accurate view.

add a heuristic to try to keep the current song after a `load'

mention -p in show usage

don't log when we abort the decoding

don't die on opus failures

simplify further the code

simplify code and don't log non-fatal errors

vorbis: fix logging and don't die on invalid files

don't leak file descriptors

don't die on failures

improve logging and remove dead comments

cleanup flac code

annotate the only two channel support for opus

support mono mp3 (mad)