alsa: fix snd_pcm_writei usage 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.

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