plugin: Export secure_memzero() to plug-ins

plugin: Export secure_memzero() to plug-ins

The provides plug-ins with a safe and secure way to santize sensitive
information such as passwords, by re-using the secure_memzero()
implementation in OpenVPN.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20170505184622.24520-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14546.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit f018dfcc3631f165232afa3d13dc2a608bdb6ce7)