reference: rework makestep

Rework makestep to cancel accumulated offset and step with the new
offset instead of accumulating new offset first, canceling all
accumulated offset and making the step.

This avoids two large frequency changes to initiate and cancel a slew
before making the step.

local.h

@@ -189,7 +189,7 @@ extern void LCL_Finalise(void);
 /* Routine to convert the outstanding system clock error to a step and
    apply it, e.g. if the system clock has ended up an hour wrong due
    to a timezone problem. */
-extern int LCL_MakeStep(double threshold);
+extern int LCL_MakeStep(void);
 
 /* Routine to schedule a leap second. Leap second will be inserted
    at the end of the day if argument is positive, deleted if negative,