aplay: add support for G.711 A_LAW enconding in AU file format

aplay: add support for G.711 A_LAW enconding in AU file format

At the time being, aplay only supports G.711 µ-law, 8-bits PCM and
16-bits PCM encodings for the AU format while the AU format supports
many more.

In Europe the most G.711 used encoding is the A-law, which is also
part of the encodings supported by the AU file format.

Add support for that encoding in aplay.

Closes: https://github.com/alsa-project/alsa-utils/pull/297
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>