[Users] SNI and Google Mail self-signed certificate for IMAP

[Michael Schwendt]

> Further reading:
> 
> F29 Self-Contained Change: GnuTLS enables TLS 1.3 by default
> https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/4CK56YE3LZ2XESIIIZ6TWPQD32F2QWCH/
>

There's also a libetpan feature request for SNI support, albeit for OpenSSL
not gnutls. https://github.com/dinhviethoa/libetpan/issues/258
It seems to me that is where the certificate retrieval and checking is done.

Claws Mail initializes and uses gnutls separately, however, too.