mail/dovecot: upgrade to 2.3.4.1

• 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.

PR: 235523
Submitted by: pascal.christen@hostpoint.ch
MFH: 2019Q1
Security: 1340fcc1-2953-11e9-bc44-a4badb296695
Security: CVE-2019-3814