client: add second form of makestep command

The second form configures the automatic stepping, similarly to the
makestep directive. It has two parameters, stepping threshold (in
seconds) and number of future clock updates for which will be the
threshold active. This can be used with the burst command to quickly
make a new measurement and correct the clock by stepping if needed,
without waiting for chronyd to complete the measurement and update the
clock.

reference.h

@@ -157,6 +157,9 @@ extern int REF_GetOurStratum(void);
 /* Modify the setting for the maximum skew we are prepared to allow updates on (in ppm). */
 extern void REF_ModifyMaxupdateskew(double new_max_update_skew);
 
+/* Modify makestep settings */
+extern void REF_ModifyMakestep(int limit, double threshold);
+
 extern void REF_EnableLocal(int stratum);
 extern void REF_DisableLocal(void);
 extern int REF_IsLocalActive(void);