Replace more nested ifs with cond

Replace more nested ifs with cond

This is a continuation of 0db5ba4 "Replace nested ifs with cond".
* lisp/play/dunnet.el (dun-special-object, dun-inven, dun-drop):
(dun-drop-check, dun-swim, dun-break): Use when and cond where
appropriate.
(dun-examine): Fix indentation.
(dun-doverb): Use when.
(dun-read-line): Refactor.