bug#9361: 24.0.50; default value of `dired-do-chmod'

[Juri Linkov]

> I think there's a problem here, indeed.  It's a bad idea to take an
> empty input to mean a certain default permission---especially when that
> default permission is not displayed in the prompt.
>
> The following patch makes dired-do-chmod demand a non-empty input.  The
> precomputed permission is still available in the M-n `future history'.

There are other demands that dired commands should not use `read-string'
that returns the default value for an empty input.  Please see
http://lists.gnu.org/archive/html/emacs-devel/2011-07/msg01200.html

So we should either change `read-string' (backward-incompatible change)
or to change `dired-mark-read-string' not to use `read-string'.

Another solution is to add a new global variable that changes the default
behavior of `read-string' and let-bind it in `dired-mark-read-string'.

> +     (while (equal result "")
> +       (setq result
> +         (read-from-minibuffer prompt nil nil
> +                               nil nil default)))

I think it should tell the user what's wrong, like `read-number' does
with "Please enter a number."

Too bad that currently semantically similar functions
`read-string' and `read-number' differ significantly
WRT handling an empty input and default values.