bug#31094: Code for second solution

[Eli Zaretskii]

> Date: Sun, 8 Apr 2018 09:17:01 -0400
> From: Boruch Baum <boruch_baum@gmx.com>
> Cc: 31094@debbugs.gnu.org
> 
> The first code snippet already only changes behavior when the function
> is called with either two or three universal prefix args.

I was under the impression that the first variant was not what you were
promoting.

> The second snippet is controlled by a boolean defcustom variable,
> defaulting to the current long print-out behavior.

That sounds sub-optimal to me: completely changing the behavior of a
command given an option is not the best UI, IMO.  Two different
commands are better.