ALSA' snd_pcm_poll_descriptors_count() can fail with -EPIPE after an underrun (e.g. after a pause), so we can't call it directly like this. It's also a bit sloppy to always call audio_nfds(), so instead save the (initial) value returned and pass it to the various functions. Issue reported and fix tested by phoebos (ben [at] bvnf [dot] space), thank you!

This hides the libsndio bits behind a small audio_* API for which in the future we may provide multiple implementations.