Implement --client-crresponse script options and plugin interface

Implement --client-crresponse script options and plugin interface

This allows scripts and plugins to parse/react to a CR_RESPONSE message

Patch V2: doc fixes, do not put script under ENABLE_PLUGIN
Patch V3: rebase
Patch V4: fix else branch of the verify_crresponse_script function
Patch V5: unify message when unable to create/write crresponse file

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Heiko Hund <heiko@ist.eigentlich.net>
Message-Id: <20220824110930.73009-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25089.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>