channel map changes

In order to support broken/incorrect/non-standard wav files, I have made a couple changes. The first was to detect use of either back-left/back-right or side-left/side-right as the stereo surround channels. The second was to allow the user to specify the audio coding mode and presence of the LFE channel on the commandline (e.g. "-acmod 7 -lfe 1"). Only the number of channels must match the specified settings.