security/openvpn-devel: upgrade port to git commit fafb05f6f3 (2023-03-31)

This brings openvpn-devel to the current HEAD of the development tree,
which is still very close to OpenVPN 2.6.3.

Adjust config handling of option "DCO" to match upstream changes - that
is, DCO is enabled by default, so we pass "--disable-dco" to configure
now if the option is unchecked.