plugin: Export base64 encode and decode functions

plugin: Export base64 encode and decode functions

This patch builds on the "Export secure_memzero() to plug-ins" patch and
adds export of openvpn_base64_encode() and openvpn_base64_decode()

This also ships with a very simple plug-in which demonstrates how to use
the new exported functions.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20170505214624.11675-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14558.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 6690769f78bbfb889fef2a54088d979896c87d51)