Move code from the ALSA output plugin.

For 32 bit DSD-over-USB support.

Prepare for removing SAMPLE_FORMAT_DSD_OVER_USB.

Converts padded 24 bit samples to packed 24 bit samples.  Will replace
the packed S24 sample format, which is not used internally.

Delete the now-unused pcm_byteswap library, and optimize the
pcm_export_state object.