dialog: set lower limit for dlg_ka_interval parameter to 30 sec

- avoid many keepalives in shorter interval than retransmission timeout
  could be
- an improvement would be to export the fr_timer value from tm and have
  condition based on it
- reported by FS#417, Bill Radis

modules/dialog/dialog.c

@@ -452,6 +452,12 @@ static int mod_init(void)
 {
 	unsigned int n;
 
+	if(dlg_ka_interval!=0 && dlg_ka_interval<30) {
+		LM_ERR("ka interval too low (%d), has to be at least 30\n",
+				dlg_ka_interval);
+		return -1;
+	}
+
 	dlg_event_rt[DLG_EVENTRT_START] = route_lookup(&event_rt, "dialog:start");
 	dlg_event_rt[DLG_EVENTRT_END] = route_lookup(&event_rt, "dialog:end");
 	dlg_event_rt[DLG_EVENTRT_FAILED] = route_lookup(&event_rt, "dialog:failed");