Introduce CRYPT_SLOT_UNBOUND keyslot status for LUKS2.

A keyslot not bound to any segment can store any key for any purpose.

To easily check slot status, new enum value is introduced.
This status is valid only for LUKS2, so the functions are backward compatible
with LUKS1.

python/pycryptsetup.c

@@ -512,6 +512,8 @@ static PyObject *CryptSetup_killSlot(CryptSetupObject* self, PyObject *args, PyO
 	case CRYPT_SLOT_INVALID:
 		PyErr_SetString(PyExc_ValueError, "Invalid slot");
 		break;
+	default:
+		break;
 	}
 
 	return NULL;