PAM: create pre-auth indicator file
If pam_cert_auth is enabled the PAM responder will automatically create
the pre-authentication indicator file to tell pam_sss to do the pre-auth
step to find out about the available authentication methods.
Related to https://pagure.io/SSSD/sssd/issue/3500
Reviewed-by: Fabiano FidĂȘncio <fidencio@redhat.com>