security/openvpn-devel: upgrade port to git commit df4863aa0e (2025-06-27)

This commit brings the port to "openvpn 2.7_alpha2" with a few extra
bugfixes.

There is one significant change here: the "old way" to update DNS
config via "--up contrib/pull-resolv-conf/client.up" and plugin-down-root.sh
has been replaced by a built-in mechanism (--dns-updown $script),
which is no loger "contributed" but maintained upstream, and brings
its own privilege separation / down-root mechanism. The old
contrib/pull-resolv-conf scripts have been removed from the upstream
distribution.

The port-specific "openvpn-client" script has been adjusted to do
the same thing ("run a client config with DNS setup") with the new
mechanism.

Besides this, this update has a number of minor bugfixes and improvements
to documentation, logging and testing infrastructure.

PR: 287892