audio: move to decoder wrapper

Use the decoder wrapper that was introduced for video. This removes all
code duplication the old audio decoder wrapper had with the video code.

(The audio wrapper was copy pasted from the video one over a decade ago,
and has been kept in sync ever since by the power of copy&paste. Since
the original copy&paste was possibly done by someone who did not answer
to the LGPL relicensing, this should also remove all doubts about
whether any of this code is left, since we now completely remove any
code that could possibly have been based on it.)

There is some complication with spdif handling, and a minor behavior
change (it will restrict the list of codecs to spdif if spdif is to be
used), but there should not be any difference in practice.

common/common.h

@@ -58,14 +58,6 @@ enum stream_type {
     STREAM_TYPE_COUNT,
 };
 
-enum {
-    DATA_OK     = 1,        // data is actually being returned
-    DATA_WAIT   = 0,        // async wait: check state again after next wakeup
-    DATA_AGAIN  = -2,       // repeat request (internal progress was made)
-    DATA_STARVE = -1,       // need input (might require to drain other outputs)
-    DATA_EOF    = -3,       // no more data available
-};
-
 extern const char mpv_version[];
 extern const char mpv_builddate[];
 extern const char mpv_copyright[];