Replace movement variables with an alist and accessor

Replace movement variables with an alist and accessor

* lisp/play/dunnet.el (north, south, east, west, northeast, southeast):
(northwest, southwest, up, down, in, out): Remove.
(dun-movement-alist): New constant.
(dun-movement): New function.
(dun-n, dun-s, dun-e, dun-w, dun-ne, dun-se, dun-nw, dun-sw, dun-up):
(dun-down, dun-in, dun-out): Use a symbol for indicating movement.
(dun-move, dun-special-move): Translate movement symbol to an
enumeration.