Updated interface protocol. Fixed M2 bug.

- Updated interface protocol to play nicer with interface programs. All
Grbl responses beginning with '$' signifies a setting. Bracketed '[]'
responses are feedback messages containing either state, parameter, or
general messages. Chevron '<>' response are from the real-time status
messages, i.e. position.

- M2 Program end command was causing a system alarm. Fixed. Thanks
@blinkenlight !

planner.c

@@ -327,10 +327,11 @@ uint8_t plan_check_full_buffer()
   return(false);
 }
 
-// Block until all buffered steps are executed.
+// Block until all buffered steps are executed or in a cycle state. Works with feed hold
+// during a synchronize call, if it should happen. Also, waits for clean cycle end.
 void plan_synchronize()
 {
-  while (plan_get_current_block()) { 
+  while (plan_get_current_block() || sys.state == STATE_CYCLE) { 
     protocol_execute_runtime();   // Check and execute run-time commands
     if (sys.abort) { return; } // Check for system abort
   }