Ensure only CBC, CFB, OFB and AEAD ciphers are considered valid data ciphers

Ensure only CBC, CFB, OFB and AEAD ciphers are considered valid data ciphers

Make sure cipher_valid only considers these four operations as valid.
This fixes that something like --data-ciphers  AES-256-GCM:AES-128-CCM
will start but later fail when trying to use the CCM cipher.

We say "a supported AEAD" mode in our error since CCM is also an AEAD mode
but one we don't support, unlike GCM.

Patch v2: add the indication if the cipher was optional into the message

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20221010155515.1687151-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25379.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>