Fix dired switch (that contain quotes and spaces) parsing

* lisp/files.el (insert-directory): Tokenize shell strings
correctly (bug#18875).

lisp/files.el

@@ -7463,7 +7463,7 @@ normally equivalent short `-D' option is just passed on to
 			      (unless (equal switches "")
 				;; Split the switches at any spaces so we can
 				;; pass separate options as separate args.
-				(split-string-and-unquote switches)))
+				(split-string-shell-command switches)))
 			    ;; Avoid lossage if FILE starts with `-'.
 			    '("--")
 			    (list file))))))