bug#24518: 25.2.50; dired-mark-extension with prefix arg fails

[Eli Zaretskii <eliz@gnu.org>]

> From: Tino Calancha <tino.calancha@gmail.com>
> Date: Sun, 25 Sep 2016 02:14:14 +0900 (JST)
> cc: Andreas Schwab <schwab@linux-m68k.org>, tino.calancha@gmail.com, 
>     24518@debbugs.gnu.org
> 
> > I believe my recent commit fixed all of the above
> Yeah, you fixed something, and we are exactly in the same point
> that before fixing such thing.  We didn't get something better.

We don't have this command signal an error anymore, do we?  I think
that's a definite improvement.

Besides, you yourself told me to fix the bug, so I did.

> The point here is that a Dired user would expect/wish that the
> interactive call behaves differently.  I do.
> Do you know how i noticed the bug?
> Because i assumed that a prefix argument will unmark as other
> Dired marks, and i got an error (!).

But it isn't true that every Dired command that marks files will
unmark them if invoked with an argument.  E.g., dired-mark doesn't.
So this doesn't look like a universal convention in Dired, only a
frequent feature.

OTOH, since Dired allows the user to use different marker characters
in other places, IMO it is not very reasonable to exempt this
particular command.  Especially since it did have a (semi-broken) way
of doing that.