Implement optional cipher in --data-ciphers prefixed with ?

Implement optional cipher in --data-ciphers prefixed with ?

This allows to use the same configuration multiple platforms/ssl libraries
and include optional algorithms that are not available on all platforms

For example "AES-256-GCM:AES-128-GCM:?CHACHA20-POLY1305" can be used to
emulate the default behaviour of OpenVPN 2.6.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20211201180727.2496903-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23279.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 766044507497c41f0319159c37992788ecb681e6)