Fix certificate serial number export

Fix certificate serial number export

    contrib/OCSP_check/OCSP_check.sh:
             New barebone script to demonstrate how to use $tls_serial_{n}
             to perform simple OCSP queries using OpenSSL command line
             "openssl ocsp". Minimal sanity checks to fail if user tries to
             use it without customizing.

    openvpn.8:
             Added some notes about $tls_serial_{n} format and usage to the
             existing description.

    ssl.c:
             correctly manage and export serial numbers of any size (as
             parsed by OpenSSL) into the environment. Set to empty string
             in case of errors, as 0 and negative numbers are all possible
             (although illegal) certificate serial numbers. Use an OpenSSL
             BIO object to do the job. Conforms to coding style guidelines.

    See the discussion at

    http://article.gmane.org/gmane.network.openvpn.devel/3588

    for more details.

Signed-off-by: Davide Brini <dave_br@gmx.com>
Signed-off-by: David Sommerseth <dazo@users.sourceforge.net>
Acked-by: David Sommerseth <dazo@users.sourceforge.net>