output/httpd: reject some well-known request paths

Return `404 not found` for some common well-known paths, as clients requesting them usually do that automatically and don't expect endless audio stram.

Closes #572

src/output/plugins/httpd/HttpdClient.hxx

@@ -83,6 +83,11 @@ class HttpdClient final
 	 */
 	bool head_method = false;
 
+	/**
+	 * Should we reject this request?
+	 */
+	bool should_reject = false;
+
 	/* ICY */
 
 	/**