Log Message:
Add a MESSAGE.common to tell user to install a libao plugin.
I used MESSAGE.common so a MESSAGE.${OPSYS} could also be added.

Bump PKGREVISION.

Clarify DESCR and do not mention all the outputs it supports as
this may make it appear it is works without installing another
plugin.

TODO: What about NULL, WAV, IRIX, and ALSA?