Hard limits code minor updates.

- Fixed a bug that would not disable the steppers if a user issues a
system abort during a homing cycle.

- Updated the hard limit interrupt to be more correct and to issue a
shutdown for the right situations when the switch has been triggered.

- Added a status message to indicate to the user what happened and what
to do upon a hard limit trigger.

protocol.h

@@ -29,6 +29,7 @@
 #define STATUS_MODAL_GROUP_VIOLATION 5
 #define STATUS_INVALID_COMMAND 6
 #define STATUS_SETTING_DISABLED 7
+#define STATUS_HARD_LIMIT 8
 
 #define LINE_BUFFER_SIZE 50