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.

serial.c

@@ -166,7 +166,7 @@ ISR(USART_RX_vect)
     case CMD_CYCLE_START:   sys.execute |= EXEC_CYCLE_START; break; // Set as true
     case CMD_FEED_HOLD:     sys.execute |= EXEC_FEED_HOLD; break; // Set as true
     case CMD_RESET:
-      sys.alarm |= EXEC_ALARM; // Set alarm to allow subsystem disable for certain settings.
+      sys.execute |= EXEC_ALARM; // Set alarm to allow subsystem disable for certain settings.
       
       // TODO: When Grbl system status is installed, set position lost state if the cycle is active.