mail/mpop: Update to 1.4.12

Adds support for the libtls library (provided by the
LibreSSL project).

Changelog: https://marlam.de/mpop/news/

PR: 252145
Submitted by: Neel Chauhan <neel@neelc.org> (maintainer)
Approved by: arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D27779