Support options with embedded whitespace in 'dired-listing-switches'

* lisp/dired.el (dired-listing-switches): Document how to quote
options with embedded whitespace.

* lisp/files.el (insert-directory): Use split-string-and-unquote
to support dired-listing-switches that specify command-line
options with embedded spaces.  (Bug#25485)

lisp/dired.el

@@ -59,6 +59,10 @@
 May contain all other options that don't contradict `-l';
 may contain even `F', `b', `i' and `s'.  See also the variable
 `dired-ls-F-marks-symlinks' concerning the `F' switch.
+Options that include embedded whitespace must be quoted
+like this: \\\"--option=value with spaces\\\"; you can use
+`combine-and-quote-strings' to produce the correct quoting of
+each option.
 On systems such as MS-DOS and MS-Windows, which use `ls' emulation in Lisp,
 some of the `ls' switches are not supported; see the doc string of
 `insert-directory' in `ls-lisp.el' for more details."

lisp/files.el

@@ -6582,7 +6582,7 @@ 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 switches)))
+				(split-string-and-unquote switches)))
 			    ;; Avoid lossage if FILE starts with `-'.
 			    '("--")
 			    (progn