bug#31946: 27.0.50; The NSM should warn about more TLS problems

[Noam Postavsky]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> There are also more protocol stuff we should warn about on various
> levels.  These should be on `high':

>>            "https://dh-small-subgroup.badssl.com/"        ;; fail
>>            "https://dh-composite.badssl.com/"             ;; fail

So these ones seem kind of problematic, as alluded to on emacs-devel.
It doesn't look like gnutls has an API to get or check the value of the
DH primes (calc-prime-test bails out when given a 1024 bit prime, so we
definitely need library support for this).

https://lists.gnu.org/archive/html/emacs-devel/2018-06/msg00805.html
https://lists.gnu.org/archive/html/emacs-devel/2018-06/msg00807.html