reference: add UpdateOnce and PrintOnce modes

reference.c

@@ -732,6 +732,25 @@ special_mode_sync(int valid, double offset)
 
       end_ref_mode(1);
 
+      break;
+    case REF_ModeUpdateOnce:
+    case REF_ModePrintOnce:
+      if (!valid) {
+        LOG(LOGS_WARN, LOGF_Reference, "No suitable source for synchronisation");
+        end_ref_mode(0);
+        break;
+      }
+
+      step = mode == REF_ModeUpdateOnce;
+
+      LOG(LOGS_INFO, LOGF_Reference, "System clock wrong by %.6f seconds (%s)",
+          -offset, step ? "step" : "ignored");
+
+      if (step)
+        LCL_ApplyStepOffset(offset);
+
+      end_ref_mode(1);
+
       break;
     case REF_ModeIgnore:
       /* Do nothing until the mode is changed */

reference.h

@@ -43,6 +43,8 @@ extern void REF_Finalise(void);
 typedef enum {
   REF_ModeNormal,
   REF_ModeInitStepSlew,
+  REF_ModeUpdateOnce,
+  REF_ModePrintOnce,
   REF_ModeIgnore,
 } REF_Mode;