mail/dovecot: upgrade to 2.3.9.3

Changelog:

• CVE-2020-7046: Truncated UTF-8 can be used to DoS submission-login and lmtp processes.
• CVE-2020-7957: Specially crafted mail can crash snippet generation.

MFH: 2020Q1
Security: CVE-2020-7046
Security: CVE-2020-7957
Security: 74db0d02-b140-4c32-aac6-1f1e81e1ad30