HUGE cleanings in audio output:
- removed AOUT_INTF_*_FIFO, which where not used
- factorized S16StereoPlay, U8MonoPlay, and so on.., in a single
  function aout_FillBuffer (in aout_common.c)
  (unsigned 8bit may be broken, but I have no stream to test with :(

- last step to be done: factorize aout_s8, aout_s16, ... thanks to a macro