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 discovered with an error in the log.

This is a backport of commit 282ddbac54f8d4923844f699 (master), taking
the different man-page format into account.  The code change is the same.

CVE: 2022-0547
Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220315155344.37787-3-openvpn@sf.lists.topphemmelig.net>
URL: https://www.mail-archive.com/search?l=mid&q=20220315155344.37787-3-openvpn@sf.lists.topphemmelig.net
Signed-off-by: Gert Doering <gert@greenie.muc.de>