OPNsense 24.7 - OpenVPN breakage due to DCO being enabled by default / add optional DCO support

[AdSchellevis]

Important notices

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• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

Describe the bug

When using FreeBSD 14.1, our OpenVPN seems to misbehave as DCO is enabled by default. To lower the risk at upgrades, we likely should disable DCO by default and try to figure out why it breaks next.

Suggested workaround

Toggle --disable-dco by default on our end. (https://openvpn.net/community-resources/reference-manual-for-openvpn-2-6/)

[NielsPiersma]

Please disable DCO. DCO is not supported for /30 networks.

[AdSchellevis]

@NielsPiersma for 24.7 it will be optional, we're certainly not automatically going to migrate tun setups to dco ones knowing the constraints that apply (https://community.openvpn.net/openvpn/wiki/DataChannelOffload/Features , https://github.com/OpenVPN/openvpn/blob/master/README.dco.md )