Improved homing cycle. New settings: homing enable/rates, debounce and step idle lock time.

- Homing cycle will now cycle twice (spec more/less in config) to
improve repeatability and accuracy by decreasing overshoot.

- New Grbl settings added: Enable/disable homing cycles, homing seek
and feed rates, switch debounce delay, and stepper idle lock time.

- Please note that these settings may change upon the next push, since
there will be more added soon. Grbl *should* not re-write your old
settings, just re-write the new ones. So, make sure you keep these
written down somewhere in case they get lost from a code bug.

- Refactored settings migration to be a little smaller and managable
going forward.

protocol.c

@@ -57,6 +57,8 @@ static void status_message(int status_code)
       printPgmString(PSTR("Modal group violation\r\n")); break;
       case STATUS_INVALID_COMMAND:
       printPgmString(PSTR("Invalid command\r\n")); break;
+      case STATUS_SETTING_DISABLED:
+      printPgmString(PSTR("Grbl setting disabled\r\n")); break;
       default:
       printInteger(status_code);
       printPgmString(PSTR("\r\n"));