Fixed homing fail alarm handling. Re-integrated software debouncing.

- [bug] Fixed a homing fail issue, where the alarm was not being set
right, not cleared correctly. It would report the wrong code and enter
an infinite alarm loop. This was due to how alarm codes were altered a
while back. Now updated and fixed to show the right codes.

- [feature] Re-installed optional software debouncing for hard limit
switches. By request.

grbl/motion_control.c

@@ -359,8 +359,9 @@ void mc_reset()
     // violated, by which, all bets are off.
     if ((sys.state & (STATE_CYCLE | STATE_HOMING | STATE_JOG)) ||
     		(sys.step_control & (STEP_CONTROL_EXECUTE_HOLD | STEP_CONTROL_EXECUTE_SYS_MOTION))) {
-      if (sys.state == STATE_HOMING) { system_set_exec_alarm(EXEC_ALARM_HOMING_FAIL_RESET); }
-      else { system_set_exec_alarm(EXEC_ALARM_ABORT_CYCLE); }
+      if (sys.state == STATE_HOMING) { 
+        if (!sys_rt_exec_alarm) {system_set_exec_alarm(EXEC_ALARM_HOMING_FAIL_RESET); }
+      } else { system_set_exec_alarm(EXEC_ALARM_ABORT_CYCLE); }
       st_go_idle(); // Force kill steppers. Position has likely been lost.
     }
   }