openvpn: T8264: adopt a pre-created DCO interface properly

VyOS creates the "ovpn" device whenever the daemon cannot - a client whose
server is not reachable yet never opens its tun - so OpenVPN has to adopt a
device it did not make. It takes the device but not its ifindex today, so
dco_new_peer() fails, the daemon exits and deletes the interface on its way
out. systemd starts it again and the second attempt creates a device of its
own, which hides the whole thing behind a tunnel that does come up.

Carry Ralf's fix until it is released: it resolves the ifindex on the
-EEXIST path, refuses a device whose operating mode does not match, and
leaves a pre-existing interface alone on close.

https://gerrit.openvpn.net/c/openvpn/+/1734 (patchset 2)

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