DCO on OpenVPN should be unavailable for TCP

[waka324]

Spent an unfortunate amount of time trying to enable DCO for a TCP OpenVPN Server in Opnsense.

There either needs to be a prominent note, error log entry, or configuration validation error thrown when trying to use DCO with TCP connections.

[Monviech (Cedrik)]

Could you open a github issue for that?

https://github.com/opnsense/core/issues