Previously, `pam_unix.so` was `required` to set PAM_AUTHTOK so that
dependent pam modules (such as gnome keyering) could use the password
(for example to unlock a keyring) upon login of the user. This however
broke any additional auth providers (such as AD or LDAP): for any
non-local user `pam_unix.so` will not yield success, thus eventually the
auth would fail (even the following auth providers were actually
executed, they could not overrule the already failed auth).
This change replaces `required` by `optional`. Therefore, the
`pam_unix.so` is executed and can set the PAM_AUTHTOK for the following
optional modules, _even_ if the user is not a local user. Therefore, the
gnome keyring for example is unlocked both for local and additional
users upon login, and login is working for non-local users via
LDAP/AD.
305b633423