Feature: Expose TLS status in <listener> tags.

This exposes the TLS status as <tls> tag in the <listener> tag.
The tag contains a bool ("true" or "false"). TLS mode, version and
other parameters are out of the scope of this tag and may be added
as independed tags in later commits.

src/admin.c

@@ -697,6 +697,8 @@ static inline xmlNodePtr __add_listener(client_t        *client,
     if (client->role)
         xmlNewChild(node, NULL, XMLSTR("role"), XMLSTR(client->role));
 
+    xmlNewChild(node, NULL, XMLSTR("tls"), XMLSTR(client->con->ssl ? "true" : "false"));
+
     return node;
 }