security/openvpn: bugfix update to 2.7.6

This is not security-relevant on FreeBSD, one security fix affects
Windows, the other one only affects builds against mbedTLS (which is not
currently supported by the port), and a third one (second bullet point)
turned out to not be exploitable.

Note that the --dev option defaults to "tun" (if not specified).

Note that --ping and --keepalive settings are now capped at 24 h.

TCP-based configurations now always enable TCP_NODELAY.

Changelog: https://github.com/OpenVPN/openvpn/releases/tag/v2.7.6
MFH: 2026Q3
Pull Request: https://github.com/freebsd/freebsd-ports/pull/584