admin_ssl_ca_cert reads the wrong certificate

The admin_ssl_ca_cert parameter uses the value of admin_ssl_cert as the client verification certificate. It also appears to be that libmicrohttpd by default doesn't verify that the client certificates are valid even if MHD_OPTION_HTTPS_MEM_TRUST is passed (unlike OpenSSL).

In addition, the CA parameter should not be mandatory as it is not required to enable TLS.