We will need to pass the source field to the keyserver to distinguish between different log-in scenarios (task).
This diff adds this field to the strict interface but allows an undefined source (which will happens on logging from the login form).
A proper type for login from form will be added later on and this field will become mandatory.

1. Modify the code to pass the source field.
2. Check if validation pass for both undefined (e.g. login from form) and defined source (e.g. cookie expired).