Notes:

- The variable value represents the PulseAudio sink index.
  It is not a member of the AOUT_VAR_ enumeration.

- There is no configuration item. PulseAudio normally remembers the
  last sink VLC used. So I think there is no need value in writing
  code to duplicate that effort.

- I am not sure if we need more locking inside the variable callback.
  This needs to be clarified with PulseAudio guys.