The helm-walk-directory version using file-name-all-completions.

* helm-utils.el (helm-walk-directory): Use file-name-all-completions.
* helm-utils.el (helm-walk-directory): Fix long lines.
* helm-utils.el (helm-walk-directory): Fix bug with infinite loop on symlinks.
* helm-utils.el (helm-walk-directory): Call directory-file-name only once.