M78 now allows stats reset using the S78 argument

Marlin/Marlin_main.cpp

    * M78: Show print statistics
    */
   inline void gcode_M78() {
-    print_job_timer.showStats();
+    // "M78 S78" will reset the statistics
+    if (code_seen('S') && code_value_short() == 78)
+      print_job_timer.initStats();
+    else print_job_timer.showStats();
   }
 #endif