Disable DCO when TLS mode is not used

Disable DCO when TLS mode is not used

This disables DCO in both --secret mode and when no encryption/TLS is
used. Also aligns the message with the deprecation warning we have in
place.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221210134427.1433419-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25641.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit a666668f064c7ff57cdebb3afceb72e1263a3ba9)