tool_operate: Add http code 408 to transient list for --retry

- Treat 408 request timeout as transient so that curl will retry the
  request if --retry was used.

Closes #2925

docs/cmdline-opts/retry.d

@@ -6,7 +6,7 @@ Help: Retry request if transient problems occur
 If a transient error is returned when curl tries to perform a transfer, it
 will retry this number of times before giving up. Setting the number to 0
 makes curl do no retries (which is the default). Transient error means either:
-a timeout, an FTP 4xx response code or an HTTP 5xx response code.
+a timeout, an FTP 4xx response code or an HTTP 408 or 5xx response code.
 
 When curl is about to retry a transfer, it will first wait one second and then
 for all forthcoming retries it will double the waiting time until it reaches

src/tool_operate.c

@@ -1631,6 +1631,7 @@ static CURLcode operate_do(struct GlobalConfig *global,
                 curl_easy_getinfo(curl, CURLINFO_RESPONSE_CODE, &response);
 
                 switch(response) {
+                case 408: /* Request Timeout */
                 case 500: /* Internal Server Error */
                 case 502: /* Bad Gateway */
                 case 503: /* Service Unavailable */