Add CURLE_SSL_CLIENTCERT error #6721
I don't mind adding a new error code for this condition. But...
I think this error code needs to also be supported at least for OpenSSL to be acceptable (and preferably for even more). As a I said before: Secure Transport is more or less deprecated by Apple themselves now and adding a global new curl error code only supported in that niche backend I think isn't meeting the bar we should set for new error codes.
@gvollant If the mutual TLS authentication set on both client and server, client does not give its certificates, the error will be return. That is our use case and at that point client asks end user to select certificates for mutual handshake. (in case of client aware of multiple certificates, client does not know which one to provide, hence end users interaction is needed.)