bug#5416: ARG of dired-hide-all is unused (and is the raw prefix arg)

emacs -Q

The argument of dired-hide-all, ARG, which is totally unused.

Worse, ARG is given its value by the prefix arg:
(interactive "P")

So something must have been foreseen for users in terms of a prefix arg, but it
was apparently not implemented.

In GNU Emacs 23.1.91.1 (i386-mingw-nt5.1.2600)
 of 2010-01-02 on PRETEST
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (3.4)'