clientlog: add support for KoD rate limiting

Add a third return value to CLG_LimitServiceRate() to indicate the
server should send a response requesting the client to reduce its
polling rate. It randomly selects from a fraction (configurable to 1/2,
1/4, 1/8, 1/16, or disabled) of responses which would be dropped
(after selecting responses for the leak option).

clientlog.h

@@ -40,6 +40,7 @@ typedef enum {
 typedef enum {
   CLG_PASS = 0,
   CLG_DROP,
+  CLG_KOD,
 } CLG_Limit;
 
 extern void CLG_Initialise(void);