snd_pcm_writei works in terms of audio frames, not buffer (bytes) length like snd_write does, so account for that. To do so, prepare a `bpf' (bytes per frame) multiplier in the configure stage and use it to convert back and forward between the buffer length and the number of frames. While here, also use snd_pcm_avail_update() to know how many frames may be written.

it still doesn't work correctly, audio is very distorted, but at least there's some sound.

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

I've noticed that often during the suspend/resume cycle amused could die due to a "sndio hang-up" caused by an EAGAIN. This should prevent that and make sure we retry the operation.