Stereo device support

This patch series adds support for stereo playback and capture devices.  This is normally what the user expects - stereo devices should have audio on both left and right channels.

Previously only mono was supported.  This caused confusion because audio was only flowing on the left channel if the device was stereo.

Now we open stereo devices with both channels.  Internally all processing remains mono.  This is somewhat of a hack but a step in the right direction.