security/openvpn-devel: Update to latest commit

This brings in the upstream development work of the last three months,
including a number of CVE fixes.

The previous commit erroneously bumped the date string used as the
version number to "2027". Fix that, and upgrade EPOCH to make it work.

Some maintenance on the port Makefile itself to address "portlint"
complaints and upstream changes (--enable-strict configure argument has
been dropped because the flags are always on now).

• Remove unnecessary GNU_CONFIGURE_MANPREFIX.
• Improve post-install-EXAMPLES-on.

PR: 296874
Security: CVE-2026-11771
Security: CVE-2026-12932
Security: CVE-2026-12996
Security: CVE-2026-13117
Security: CVE-2026-13122
Security: CVE-2026-13698
Sponsored by: UNIS Labs
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>
Co-authored-by: Matthias Andree <mandree@FreeBSD.org>
MFH: 2026Q3

(cherry picked from commit b7a71975ead7afe91139e6bb5650da5dff367223)