ALSA: introduce the snd-aloop module for the PCM loopback

The snd-aloop module allows redirecting of the PCM playback in the
kernel back to the user space using the standard ALSA PCM capture API.

The module also allows time synchronization with another timing source
and notifications of playback stream parameter changes.

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