(pam) work in progress: pam helper skip for non local users, for non existing users pam should proceed to the next option.

When our pam modules is used, we set local auth to disabled (locked) for our users, non local users should automatically use normal unix auth.
For this to work, we should pass PAM_USER_UNKNOWN as return value in our pam module when the user is not local to us and unauthenticated.