Implement custom HTTP header for http-proxy, and always send user-agent:

There are some patched OpenVPN versions out there without source code
(e.g. NDMVPN) that support adding custom http header.

This patch adds custom header to OpenVPN and supports the syntax that the
"in the wild" variants use.

Patch v3 also prints all custom headers with other http options in --verb 5
Patch v4 does clean up the add_proxy_header function
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1382688143-17247-1-git-send-email-arne@rfc2549.org>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7946

Signed-off-by: Gert Doering <gert@greenie.muc.de>