enhance tls-verify possibility

enhance tls-verify possibility

It should be nice to enhance tls-verify check possibilities against peer
cert during a pending TLS connection like :
- OCSP verification
- check any X509 extensions of the peer certificate
- delta CRL verification
- ...

This patch add a new "tls-export-cert" option which allow to get peer
certificate in PEM format and to store it in an openvpn temporary file.
Peer certificate is stored before tls-script execution and deleted after.
The name of the related temporary file is available under tls-verify
script by an environment variable "peer_cert".

The patch was made from OpenVPN svn Beta21 branches.

Here is a very simple exemple of Tls-verify script which provide OCSP
support to OpenVPN (with tls-export-cert option) without any OpenVPN
"core" modification :

X509=$2

openssl ocsp \
      -issuer /etc/openvpn/ssl.crt/RootCA.pem \
      -CAfile /etc/openvpn/ssl.capath/OpenVPNServeur-cafile.pem \
      -cert $peer_cert \
      -url http://your-ocsp-url
      if [ $? -ne 0 ]
      then
          echo "error : OCSP check failed for ${X509}" | logger -t
"tls-verify"
          exit 1
       fi

This has been discussed here:
<http://thread.gmane.org/gmane.network.openvpn.devel/2492>
<http://thread.gmane.org/gmane.network.openvpn.devel/3150>
<http://thread.gmane.org/gmane.network.openvpn.devel/3217>

This patch has been modified by David Sommerseth, by fixing a few issues
which came up to during the code review process.  The man page has been
updated and tmp_file in ssl.c is checked for not being NULL before calling
delete_file().

Signed-off-by: David Sommerseth <dazo@users.sourceforge.net>
Acked-by: Gert Doering <gert@greenie.muc.de>