mail/getmail6: Update to upstream 6.18.1 release

This release includes a few more changes beyond the last commit, which
was version 6.18 plus one commit.

https://github.com/getmail6/getmail6/releases/tag/v6.18.1

Reported by: Michael Büker <freebsd@michael-bueker.de> (maintainer)
Approved by: Michael Büker <freebsd@michael-bueker.de> (maintainer)