PCM - Change the hw_params determination order

PCM - Change the hw_params determination order

In snd_pcm_hw_params_choose(), set the buffer size before the period
size and time as default.  This will give more useful configuration for
most of apps, i.e. larger buffer size.

For apps that require the old behavior, now the function checks the
environment variable $LIBASOUND_COMPAT.  If this variable is set to
non-empty, the hw_params is determined in the old way, first period
then buffer sizes.

Signed-off-by: Takashi Iwai <tiwai@suse.de>