Add drop_unauthorized parameter to proxy hosts

drop_unauthorized returns 444 when a client is not authorized as opposed
to 403. It can be used with Client Certificate authorization.

backend/templates/_access.conf

@@ -2,7 +2,7 @@
 {% if access_list.clientcas.size > 0 %}
     # TLS Client Certificate Authorization
     if ($ssl_client_verify != "SUCCESS") {
-        return 403;
+        return {% if drop_unauthorized == 1 %}444{% else %}403{% endif %};
     }
 {% endif %}
     {% if access_list.items.length > 0 %}