Log serial number of revoked certificate

Log serial number of revoked certificate

In most of situations admin of OpenVPN server needs to know which
particular certificate is used by client.
In the case when certificate is OK, environment variable can be used for
that but once it is revoked, no user scripts are invoked so there is
no way to get serial number: only subject is printed in logs.

So we log certificate serial in case it is revoked.

Sponsored-by: Yandex LLC
Signed-off-by: Boris Lytochkin <lytboris@yandex-team.ru>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <55FEBF7E.3010209@yandex-team.ru>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10154

Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 767e4c56becbfeea525e4695a810593f373883cd)