Fixes the following security issues:
* CVE-2019-3814: If imap/pop3/managesieve/submission client has
trusted certificate with missing username field
(ssl_cert_username_field), under some configurations Dovecot
mistakenly trusts the username provided via authentication instead
of failing.
* ssl_cert_username_field setting was ignored with external SMTP AUTH,
because none of the MTAs (Postfix, Exim) currently send the
cert_username field. This may have allowed users with trusted
certificate to specify any username in the authentication. This bug
didn't affect Dovecot's Submission service.
For more details, see the announcement:
https://www.dovecot.org/list/dovecot-news/2019-February/000394.html
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
a30d577a4b