bug#51733: 27.1; Detect impossible email addresses better

[Eli Zaretskii]

> Date: Tue, 18 Jan 2022 20:48:46 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 51733@debbugs.gnu.org
> 
> These two tests seem to reveal a bug in the implementation:
> 
>   (should (textsec-name-suspicious-p
>            "\N{LEFT-TO-RIGHT MARK}\N{LEFT-TO-RIGHT MARK}Lars Ingebrigtsen"))
>   (should (textsec-name-suspicious-p
>                "\N{LEFT-TO-RIGHT MARK}\N{RIGHT-TO-LEFT MARK}\N{LEFT-TO-RIGHT 
> MARK}\N{RIGHT-TO-LEFT MARK}\N{LEFT-TO-RIGHT MARK}Lars Ingebrigtsen")))
> 
> LRM and RLM are stateless controls, so they shouldn't be flagged as
> suspicious, AFAIU.

I think I get it now: it's because of textsec-suspicious-nonspacing-p,
which forbids consecutive nonspacing characters, right?  But then I
don't think it's correct to consider Cf characters for that purpose:
UTS#39 explicitly talks about nonspacing _marks_, i.e. Mn and Me
characters.  Where did you see Cf and Cc as well?