* helm-files.el: Fix boring-file in *findutils (#579).
(helm-boring-file-regexp-list): Build it from `completion-ignored-extensions'.
(helm-ff-skip-boring-files): Document.
(helm-source-findutils): Don't skip boring files from transformer.
(helm-find-shell-command-fn): Build command from `find-cmd'.
* helm-files.el (helm-find-shell-command-fn): require find-cmd.