If a user has an empty service name, use "mysql" as default.
Authentication data was only updated inside get_pam_user_services() if no service
was found. It was possible that the PAM service changed but the old service
would be used for authenticating, causing a false negative.
Now, the auth data is updated outside the function if authentication fails for
any reason. The new service data is compared to the old and if equal, password
check is not attempted again. This gives a false negative only if user password
has changed after the previous attempt.
Also, fixed some comments.
e918810a4f