polarssl: add --verify-client-cert optional support

polarssl: add --verify-client-cert optional support

This adds support for the --verify-client-cert optional option in PolarSSL
builds, as was earlier added for OpenSSL builds by Jan-Just Keijser.

This patch also adds an additional sanity check that this option may only
be used in combination with some other authentication method, and changes
the warning message about this option to be displayed only once on startup,
instead of for each connecting client.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Jan Just Keijser <janjust@nikhef.nl>
Message-Id: <1444948995-18720-3-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10288
Signed-off-by: Gert Doering <gert@greenie.muc.de>