Make AUD_init failure fatal

Failure to initialize the audio subsystem is not handled consistently.
Where it is handled it has guest visible effects, which is wrong.
We already have a "nosound" audio driver as a last resort, so trying to
proceed without an audio backend seems pointless.

Also protect against multiple calls to AUD_init so that this can be
pushed down into individual devices.

Signed-off-by: Paul Brook <paul@codesourcery.com>
