local: replace is_step_change parameter of change handler with enum

Prepare for a new change type that will be added later.

local.h

@@ -67,16 +67,21 @@ extern void LCL_GetOffsetCorrection(struct timeval *raw, double *correction, dou
    doffset : delta offset applied (positive => make local system fast
    by that amount, negative => make it slow by that amount)
 
-   is_step_change : true if change is being applied as a jump (using
-   settimeofday rather than adjtime)
+   change_type : what type of change is being applied
    
    anything : Passthrough argument from call to registration routine */
 
 
+typedef enum {
+  LCL_ChangeAdjust,
+  LCL_ChangeStep
+} LCL_ChangeType;
+
 typedef void (*LCL_ParameterChangeHandler)
      (struct timeval *raw, struct timeval *cooked,
       double dfreq,
-      double doffset, int is_step_change,
+      double doffset,
+      LCL_ChangeType change_type,
       void *anything
       );