Merge topic branch t/afs-cleanups into master

Merge topic branch t/afs-cleanups into master

A fair number of patches which clean up all parts of the audio file
selector. The most visible change is probably that error messages from
afs callbacks are sent with a proper sideband designator so that they
are written to stderr on the client side.

* refs/heads/t/afs-cleanups: (30 commits)
  Introduce afs_error().
  afs.c: Move com_select() and its callback down.
  Rename mood_switch(), mood_close(), playlist_{open/close}.
  Assume that score_open() and score_clear() always succeed.
  playlist.c: Rename playlist_info -> playlist_instance.
  mood.c: Rename struct mood to mood_instance.
  afs.c: Rename ->handler of struct callback_query to ->cb.
  Simplify and improve activate_mood_or_playlist().
  afs: Replace ->init of afs tables by table operations.
  Merge load_playlist() into playlist_open() and simplify.
  Simplify row_belongs_to_score_table().
  Remove mood.h.
  Clean up and rename change_current_mood().
  mood.c: Simplify and rename load_mood().
  mood.c: Move struct statistics into struct mood.
  afs.c: Improve activate_mood_or_playlist().
  Improve playlist_open().
  blob.c: Don't initialize table pointer in table->init().
  blob: Constify name argument of blob_get_def_by_name().
  Rename admissible_file_loop() -> score_loop().
  ...