If ntlm_auth actually uses the cred cache, it will not give a session key.
As the Wine NTLM code depends on the session key to do transport crypto, don't
attempt to use the winbind cred cache.

This completely reverts my attempted fix 7788c8ed and also parts of Rob's
888a8e27 and 8a2125f9. It does not affect the code using wine's own credential
cache.