bug#51733: 27.1; Detect impossible email addresses better

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 51733@debbugs.gnu.org
> Date: Mon, 17 Jan 2022 17:57:46 +0100
> 
> >> They then recommend comparing the value with the zero value of that
> >> system, and I'm pretty sure we don't have that.
> >
> > Why not? what do you need, exactly?
> 
> Just a thinko -- I was wondering whether we had a way to find the zero
> character, but that's just:
> 
> (- ?৪ (get-char-code-property ?৪ 'numeric-value))

That's just sheer luck, AFAIU (there are some characters with
numeric-value property that are not arranged from zero to 9), but
maybe for this particular purpose it's all that's needed.