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

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

OpenSSL 1.1 does not allow us to directly access the internal of
any data type, including EVP_PKEY. 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: <20170612134330.20971-3-logout@free.fr>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14795.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>