output: use bool for return values and flags

Don't return 0/-1 on success/error, but true/false.  Instead of int,
use bool for storing flags.

src/output_api.c

@@ -30,7 +30,7 @@ void audio_output_closed(struct audio_output *ao)
 {
 	assert(ao->open);
 
-	ao->open = 0;
+	ao->open = false;
 }
 
 bool audio_output_is_pending(const struct audio_output *ao)