bug#51733: 27.1; Detect impossible email addresses better

[Lars Ingebrigtsen <larsi@gnus.org>]

OK, I think the textsec stuff is basically 90% implemented now.  (So
according to custom, there's at least 90% left.)

The next step would be to make other packages use this.  For instance,
when shr displays a suspicious URL, it could mark it in red (and perhaps
add a warning icon), and have a tooltip that describes in which way it's
suspicious.

I think the places it would make sense to hook this machinery in would
be in:

* shr (displaying URLs and links)
* Gnus/rmail (displaying email addresses)
* Message (when responding to mail; a prompt "do you really?")
* browse-url (prompt)

Feel free to add to the list.

There should probably be a customization point?  A user option like
`warn-about-suspicious-identifiers'?  (Better name would be nice.)  And
then a utility function that would return a propertised string with the
warning, perhaps, so that all the callers don't have to do so much work.
So shr/Gnus/rmail could use
(possibly-add-warning-about-suspiciousness string) to do that, and if
the user has switched the user option off, textsec isn't loaded at all.
(Since it loads so much data, some people might prefer not to.)

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no