Feature request: allow authentication without acct_mgmt call

[euclio]

My use case is that I'm implementing a screen locker where I am only interested in authenticating the user, not checking their account validity.

[1wilkens]

That is indeed a valid use-case, that I did not explicitly consider before. I am currently thinking about how to implement that, as I want the Client to be as useful as possible for common workflows. We could add another function like authenticate_only or find another name for the current combination of authenticate and acct_mgmt.
If you have any ideas, I am open to hear them :)