mil/opensmtpd: update to 6.6.2p1 relase

This update addressed LPE and RCE vulnerabilities in OpenSMTPD (CVE-2020-7247)
https://www.openwall.com/lists/oss-security/2020/01/28/3

This vulnerability is exploitable since May 2018 (commit a8e222352f, "switch
smtpd to new grammar") and allows an attacker to execute arbitrary shell
commands, as root:

• either locally, in OpenSMTPD's default configuration (which listens on the loopback interface and only accepts mail from localhost);

• or locally and remotely, in OpenSMTPD's "uncommented" default configuration (which listens on all interfaces and accepts external mail).

PR: 243686
Reported by: authors via irc
MFH: 2020Q1
Relnotes: https://www.mail-archive.com/misc@opensmtpd.org/msg04850.html