ntp: suggest clients to increase their polling interval

When the poll value in a client request is smaller than the server's NTP
rate limiting interval, set poll in the response to the rate limiting
interval to suggest the client to increase its polling interval.

This follows ntpd as a server. No current client implementation seems to
be increasing its interval by the poll, but it may change in the future.

clientlog.c

@@ -128,6 +128,9 @@ static int cmd_leak_rate;
 /* Flag indicating whether the last response was dropped */
 #define FLAG_NTP_DROPPED 0x1
 
+/* NTP limit interval in log2 */
+static int ntp_limit_interval;
+
 /* Flag indicating whether facility is turned on or not */
 static int active;
 
@@ -309,11 +312,13 @@ CLG_Initialise(void)
   ntp_tokens_per_packet = cmd_tokens_per_packet = 0;
   ntp_token_shift = cmd_token_shift = 0;
   ntp_leak_rate = cmd_leak_rate = 0;
+  ntp_limit_interval = MIN_LIMIT_INTERVAL;
 
   if (CNF_GetNTPRateLimit(&interval, &burst, &leak_rate)) {
     set_bucket_params(interval, burst, &max_ntp_tokens, &ntp_tokens_per_packet,
                       &ntp_token_shift);
     ntp_leak_rate = CLAMP(MIN_LEAK_RATE, leak_rate, MAX_LEAK_RATE);
+    ntp_limit_interval = CLAMP(MIN_LIMIT_INTERVAL, interval, MAX_LIMIT_INTERVAL);
   }
 
   if (CNF_GetCommandRateLimit(&interval, &burst, &leak_rate)) {
@@ -606,6 +611,14 @@ void CLG_GetNtpTimestamps(int index, NTP_int64 **rx_ts, NTP_int64 **tx_ts)
 
 /* ================================================== */
 
+int
+CLG_GetNtpMinPoll(void)
+{
+  return ntp_limit_interval;
+}
+
+/* ================================================== */
+
 int
 CLG_GetNumberOfIndices(void)
 {