bug#38457: 27.0.50; dabbrev-expand regression due to message change

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: 38457@debbugs.gnu.org
> Date: Fri, 20 Dec 2019 00:16:46 +0200
> 
> So this patch implements only the option clear-timeout
> with the 2nd variant by default.

I'm very sorry to have misled you: I meant the 3rd variant, of course.
IOW, the default value of minibuffer-message-clear-timeout should not
be a number.

Other than that, the patch LGTM.  Please push, and thanks again for
all your work on these matters.