Relevant BUGIDs: 468724

Relevant BUGIDs: 468724

Purpose of commit: bugfix

Commit summary:
---------------
Legacy behavior for pam_close_session and pam_setcred was not sufficient.
Basically, it appears to be common practice for some applications to call
these functions without first calling pam_authenticate and pam_open_session
which would have frozen the auth and session module stacks.

The new behavior is to treat the returns of these secondary functions as
authoritative when navigating the stack in the absence of a chain-freezing
first set of calls.

pam_chauthtok should not benefit from this behavior, and there does not
appear to be a justification for using an event like this to freeze the
stack outright - legacy behavior did not do that.