Change playlist_new default from 4 to 10

Change src->history = playlist_new(4 /* DOCUMENT: default is max_tracks=4. */); To src->history = playlist_new(10 /* DOCUMENT: default is max_tracks=10. */);

Other servers like Shoutcast has 10, various players and webplayers has 10. Most service providers do minimal configuration changes so a default of 10 is beneficial as that is most likely what users want anyway.