Disallow usage of --server-poll-timeout in --secret key mode.

Disallow usage of --server-poll-timeout in --secret key mode.

The internal machinery wants TLS for this to work, so just add this
to the (long) list of options not allowed unless either --tls-client
or --tls-server is active.  For added sanity, add an ASSERT() call
to the place where this combination caused a NULL ptr reference, and
document the restriction.

Fix trac #373

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1432472554-24666-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9736