Implement the --tls-export-cert feature

Implement the --tls-export-cert feature

This is a re-implementation of the --tls-export-cert feature. This
was necessary to due to missing approval to re-license the old
(now removed) code. The re-implementation is based on the following
description of the feature provided by David:

  Add an option to export certificate in PEM format of the remote
  peer to a given directory.

  For example: --tls-export-cert /var/tmp

  This option should use a randomised filename, which is provided via a
  "peer_cert" environment variable for the --tls-verify script or the
  OPENVPN_PLUGIN_TLS_VERIFY plug-in hook.

Once the script or plugin call has completed, OpenVPN should delete
this file.

Change-Id: Ia9b3f1813d2d0d492d17c87348b4cebd0bf19ce2
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20240116101556.2257-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28014.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit c58c7c3c669461805956dabc703c1279fe58eeee)