M110 ang M29 rework

Add 'ok' output for G29 actually closing the file.
Handle M110 when received without linenumber/checksumm.

Marlin/Marlin_main.cpp

@@ -756,6 +756,7 @@ void loop() {
           // M29 closes the file
           card.closefile();
           SERIAL_PROTOCOLLNPGM(MSG_FILE_SAVED);
+          ok_to_send();
         }
         else {
           // Write the string from the read buffer to SD
@@ -763,7 +764,7 @@ void loop() {
           if (card.logging)
             process_next_command(); // The card is saving because it's logging
           else
-            SERIAL_PROTOCOLLNPGM(MSG_OK);
+            ok_to_send();
         }
       }
       else
@@ -4052,6 +4053,13 @@ inline void gcode_M109() {
 
 #endif // HAS_TEMP_BED
 
+/**
+ * M110: Set Current Line Number
+ */
+inline void gcode_M110() {
+  if (code_seen('N')) gcode_N = code_value_long();
+}
+
 /**
  * M111: Set the debug level
  */
@@ -5926,6 +5934,10 @@ void process_next_command() {
         gcode_M104();
         break;
 
+      case 110: // M110: Set Current Line Number
+        gcode_M110();
+        break;
+
       case 111: // M111: Set debug level
         gcode_M111();
         break;