plug-ins: Disallow multiple deferred authentication plug-ins

plug-ins: Disallow multiple deferred authentication plug-ins

The plug-in API in OpenVPN 2.x is not designed for running multiple
deferred authentication processes in parallel. The authentication
results of such configurations are not to be trusted.  For now we bail
out when this is discovered with an error in the log.

CVE: 2022-0547
Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20220313193154.9350-3-openvpn@sf.lists.topphemmelig.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23931.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Change-Id: Ie5d1496e1e00b0f1e5be741785b8022161073867