t_server: Enable DCO support

[flichtenheld]

We have some code in there to support ovpn-dco which doesn't really help since we mostly need ovpn-backports. So rework that to support both (ovpn-dco for 2.6 and ovpn-backports for 2.7+)

[flichtenheld]

Did some first testing. Test 11 is one of the few tests that actually can use DCO by default (most others enable compression or unsupported ciphers or both).

Currently fails with

2026-06-26 10:54:15 dco_new_peer: netlink reports error (-28): Operation not permitted
2026-06-26 10:54:15 dco_new_peer: failed to send netlink message: Operation not permitted (-1)
2026-06-26 10:54:15 Cannot add peer to DCO: Operation not permitted (-1)

So the container do not have the required permissions, yet.