Setup to find data corruption and general clean up

This data corruption problem is very difficult.  Just changing the code
a little bit changes whether the problem even happens and what is
affected.  I need these changes in the main branch so I can operate with
the extra debug code always available and turned on.

Everything is setup such that if M100 is turned off or DEBUG(ECHO) is
turned off, the code is not affected.   M100 has been made a little bit
more inteligent so it can display the serial command buffers in a more
meaningful way (because the data corruption seems to often times end up
in that area).

Marlin/Marlin_main.cpp

@@ -284,6 +284,7 @@
 
 #if ENABLED(M100_FREE_MEMORY_WATCHER)
   void gcode_M100();
+  void M100_dump_routine( char *title, char *start, char *end); 
 #endif
 
 #if ENABLED(SDSUPPORT)
@@ -359,7 +360,11 @@ static long gcode_N, gcode_LastN, Stopped_gcode_LastN = 0;
 uint8_t commands_in_queue = 0; // Count of commands in the queue
 static uint8_t cmd_queue_index_r = 0, // Ring buffer read position
                cmd_queue_index_w = 0; // Ring buffer write position
+#if ENABLED(M100_FREE_MEMORY_WATCHER)
+  char command_queue[BUFSIZE][MAX_CMD_SIZE];  // Necessary so M100 Free Memory Dumper can show us the commands and any corruption
+#else                                         // This can be collapsed back to the way it was soon.
 static char command_queue[BUFSIZE][MAX_CMD_SIZE];
+#endif
 
 /**
  * Current GCode Command
@@ -9561,6 +9566,10 @@ void process_next_command() {
   if (DEBUGGING(ECHO)) {
     SERIAL_ECHO_START;
     SERIAL_ECHOLN(current_command);
+    #if ENABLED(M100_FREE_MEMORY_WATCHER)
+      SERIAL_ECHOPAIR("slot:", cmd_queue_index_r);                                                
+      M100_dump_routine( "   Command Queue:", &command_queue[0][0], &command_queue[BUFSIZE][MAX_CMD_SIZE] );  
+    #endif
   }
 
   // Sanitize the current command: