Add snd_pcm_avail() and snd_pcm_avail_delay() functions. Make snd_pcm_hwsync() deprec...

Add snd_pcm_avail() and snd_pcm_avail_delay() functions. Make snd_pcm_hwsync() deprecated.

As proposed in http://mailman.alsa-project.org/pipermail/alsa-devel/2008-June/008558.html
the snd_pcm_avail() and snd_pcm_avail_delay() functions are now available
to get accurate stream position in a straight way. The snd_pcm_avail_delay()
function was added to ensure full sync between avail and delay values. It's
actually implemented using delay() + avail_update() calls but it might be
changed in future.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>