pam_sss: return PAM_AUTHINFO_UNAVAIL if sc options are set
If pam_sss is called for PAM_USER root it currently returns
PAM_USER_UNKNOWN since SSSD does not handle root. To meet the documented
behavior if one to the sc options is used pam_sss should return
PAM_AUTHINFO_UNAVAIL in this case as well.
Related to https://pagure.io/SSSD/sssd/issue/3876
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>