* sokoban.el: Style tweaks.

(sokoban-level-file): Use load-file-name to find sokoban-levels.
(sokoban-init-level-data): Avoid messing with windows.  Use eobp and dotimes.
(sokoban-display-options, sokoban-get-level-data, sokoban-init-buffer)
(sokoban-draw-score): Use dotimes.
(sokoban-add-move, sokoban-add-push): Use push.
(sokoban-goto-level): `signal' can't return.
(sokoban-mode): Use define-derived-mode.  Set vars locally.