Future-proof runout detection in M600

No longer use saved_printing, since in newer versions M600 no longer
executes within a paused state. Introduce a new internal code 'R'
to do the same.

Firmware/Marlin_main.cpp

@@ -3054,7 +3054,7 @@ static T gcode_M600_filament_change_z_shift()
 #endif
 }	
 
-static void gcode_M600(bool automatic, float x_position, float y_position, float z_shift, float e_shift, float /*e_shift_late*/)
+static void gcode_M600(bool automatic, bool runout, float x_position, float y_position, float z_shift, float e_shift, float /*e_shift_late*/)
 {
     st_synchronize();
     float lastpos[4];
@@ -3097,7 +3097,7 @@ static void gcode_M600(bool automatic, float x_position, float y_position, float
 
     // Unload filament
     if (mmu_enabled) extr_unload();	//unload just current filament for multimaterial printers (used also in M702)
-    else unload_filament(saved_printing); //unload filament for single material (used also in M702)
+    else unload_filament(runout); //unload filament for single material (used also in M702)
     //finish moves
     st_synchronize();
 
@@ -7167,7 +7167,8 @@ SERIAL_PROTOCOLPGM("\n\n");
 		float e_shift_init = 0;
 		float e_shift_late = 0;
 		bool automatic = false;
-
+		bool runout = false;
+
         //Retract extruder
         if(code_seen('E'))
         {
@@ -7225,8 +7226,13 @@ SERIAL_PROTOCOLPGM("\n\n");
 
 		if (mmu_enabled && code_seen("AUTO"))
 			automatic = true;
+        if (code_seen('R'))
+        {
+            // Code 'R' is supported internally to indicate filament change due to a runout condition
+            runout = true;
+        }
 
-		gcode_M600(automatic, x_position, y_position, z_shift, e_shift_init, e_shift_late);
+		gcode_M600(automatic, runout, x_position, y_position, z_shift, e_shift_init, e_shift_late);
 
 	}
     break;

Firmware/fsensor.cpp

@@ -531,7 +531,7 @@ void fsensor_enque_M600(){
 	eeprom_update_word((uint16_t*)EEPROM_FERROR_COUNT_TOT, eeprom_read_word((uint16_t*)EEPROM_FERROR_COUNT_TOT) + 1);
 	enquecommand_front_P(PSTR("PRUSA fsensor_recover"));
 	fsensor_m600_enqueued = true;
-	enquecommand_front_P((PSTR("M600")));
+	enquecommand_front_P((PSTR("M600 R")));
 }
 
 //! @brief filament sensor update (perform M600 on filament runout)