Curl_ossl_connect(), the Curl_gtls_connect() function does not send the user
certificate to the peer. In fact, it ignores the conn->data->set.cert field
completely, it always uses the anonymous credentials. See
http://curl.haxx.se/bug/view.cgi?id=1348930
changelog of the current development status, as one or more of these problems
may have been fixed since this was written!
+27. "libcurl built with GNUTLS ignores the SSLCERT option" - Unlike
+ Curl_ossl_connect(), the Curl_gtls_connect() function does not send the user
+ certificate to the peer. In fact, it ignores the conn->data->set.cert field
+ completely, it always uses the anonymous credentials. See
+ http://curl.haxx.se/bug/view.cgi?id=1348930
+
26. NTLM authentication using SSPI (on Windows) when (lib)curl is running in
"system context" will make it use wrong(?) user name - at least when compared
to what winhttp does. See http://curl.haxx.se/bug/view.cgi?id=1281867