Add warning when using HTTPS with mod_curl

Add warning when using HTTPS with mod_curl

mod_curl currently does not verify the authenticity of the peer's
certificate, and does not verify whether the common name on the
certificate matches the server.  This makes mod_curl initiated TLS
connections completely insecure.  We should fix this, but until we do,
we'll warn people that it's not doing what they may think it is.

ref: http://curl.haxx.se/libcurl/c/CURLOPT_SSL_VERIFYPEER.html
ref: http://curl.haxx.se/libcurl/c/CURLOPT_SSL_VERIFYHOST.html