OpenSSL: don't use direct access to the internal of SSL_CTX

OpenSSL: don't use direct access to the internal of SSL_CTX

OpenSSL 1.1 does not allow us to directly access the internal of
any data type, including SSL_CTX. We have to use the defined functions
to do so.

Compatibility with OpenSSL 1.0 is kept by defining the corresponding
functions when they are not found in the library.

Signed-off-by: Emmanuel Deloget <logout@free.fr>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <a77187a66affdba318ef70e0e218b69cdad509d1.1487368114.git.logout@free.fr>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14088.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 6554ac9fed9c5680f22aa4722e6e07ebf3aa3441)