Pass user object to preprocess function.

Currently I am trying to let users logon to ZfcUser using legacy passwords. I was thinking of doing so by using the preprocess function. However it fails in that regard.

For this to work I need to have a case distinction between old and new users. I obviously have that state in my `UserEntity` model. There is however no way to access the user object at preprocessing time.

Therefore this PR adds a second paramter to the function passing in the user object. I find the API to still be easy usable and this change is BC because passing in a callback using a single parameter is still valid.