reference: add new leap second handling modes

In addition to the system driver handling add new modes to slew or step
the system clock for leap second, or ignore it completely. This can be
configured with leapsecmode directive.

reference.h

@@ -35,6 +35,14 @@
 #include "ntp.h"
 #include "reports.h"
 
+/* Leap second handling modes */
+typedef enum {
+  REF_LeapModeSystem,
+  REF_LeapModeSlew,
+  REF_LeapModeStep,
+  REF_LeapModeIgnore,
+} REF_LeapMode;
+
 /* Init function */
 extern void REF_Initialise(void);