Also announce IV_CIPHERS as client in OpenVPN 2.4

Also announce IV_CIPHERS as client in OpenVPN 2.4

This improves compatbility to a OpenVPN 2.5 server and
allows to negotiate a different cipher than AES-128/256-GCM
without abusing the poor man's NCP support with --cipher.

We keep the IV_NCP=2 flag logic as broken as it is since 2.5 server
ignore the flag if IV_CIPHERS is set and this might break existing
2.4 setups.

Server support for IV_CIPHERS is not added since it would be quite
intrusive and users should rather upgrade to 2.5 on the server
if they want the full benefits.

This commit cherry picks a few parts of
868b200c3aef6ee5acfdf679770832018ebc7b70

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200830140736.16571-3-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20844.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>