plugin: Improve the handling of default plug-in directory

plugin: Improve the handling of default plug-in directory

OpenVPN uses a default plug-in directore, set using PLUGINDIR when
running ./configure.  If this is set, it will use $LIBDIR/openvpn/plugin.

When using --plugin, OpenVPN will load plug-ins from this directory with
the only exception if the plug-in filename is based on an absolute path.
Any other relative paths are relative to the PLUGINDIR.

This patch adds a third variant, using plug-in paths starting with '.'
In this case, OpenVPN will use the relative directory of where OpenVPN
was started, or the directory OpenVPN have changed into due to --cd
being used before the actual --plugin option.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20170127142120.10492-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13970.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>