disable DCO if --secret is specified

disable DCO if --secret is specified

P2P mode with pre-shared key is deprecated, unsecure and should NOT be
used. This said we still carry it around for a bit and we have to make
sure it does not fight with DCO.

Disable DCO at all when --secret is specified.

Github: OpenVPN/openvpn#188

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221207100201.6467-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25629.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit f5febf7f8998e425afb051cd6f965f4dd40b1659)