opensmtpd: 6.6.3p1 -> 6.6.4p1

Release notes aren't available at this time [1] it is likely to be
related to a recent mail to oss-security (either [2] or [3]).

[1] https://www.mail-archive.com/misc@opensmtpd.org/msg04888.html
[2] https://www.openwall.com/lists/oss-security/2020/02/24/5
[3] https://www.openwall.com/lists/oss-security/2020/02/24/4