Remove duplicate X509 env variables

Remove duplicate X509 env variables

Commit 13b585e8 added support for multiple X509 env variables with the
same name, but as a side effect caused these variables to pile up for
each renegotiation.  The old code would simply overwrite the old variables
(as long as an equally-long chain was used for the new session).

To stop the variables from piling up, this commit removes any old X509
env variables if we start negotiating a new TLS session.

Trac: #854

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1489047212-31994-1-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14237.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>