write: Support audio formats != 16 bit little endian.

This adds the new --sample_format option to para_write and teaches
the check_wav_task to read the sample format from the wav header
rather than assuming 16 bit little endian.

The alsa, oss and osx writers all ask the upper buffer tree nodes
for the current audio format. For para_write the check_wav task
answers this query by using the value given at the command line,
looking at the wav header, or using 16 bit little endian as the
default answer.

For para_audiod, the current decoder answers this query. In this
case the audio format is either 16 bit little endian or 16 bit
big endian, depending on the endianness of the machine.