alsa: Select on the alsa poll fd.

This teaches the post_select method of the alsa writer to get a control
file descriptor from the alsa library via snd_pcm_poll_descriptors().
This file descriptor becomes readable when a buffer period has passed
and new samples can be written to the alsa handle. We add the fd to
the read fd set so that the select() call of the main scheduler loop
returns just in time.

This is more precise and simpler than the previous approach to compute
the time until a buffer underrun occurs.