Update openvpn_plugin_func_v2 to _v3 in sample-plugins/defer/simple.c

Update openvpn_plugin_func_v2 to _v3 in sample-plugins/defer/simple.c

This isn't strictly required, but it modernizes the functions used.
This change makes _open the same parameter form as _func (for better
parallelism in function writing) and includes a check for the correct
struct version, as recommended by openvpn-plugin.h
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1611778909-20630-1-git-send-email-gcox@mozilla.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21507.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 7d1361c18f38d6301b4d558578c73e74f6597927)