pam_sss: refactoring, use struct cert_auth_info
    
    Similar as in the PAM responder this patch replaces the individual
    certificate authentication related attributes by a struct which can be
    used as a list. With the pam_sss can handle multiple SSS_PAM_CERT_INFO
    message and place the data in individual list items.
    
    If multiple certificates are returned before prompting for the PIN a
    dialog to select a certificate is shown to the users. If available a GDM
    PAM extension is used to let the user choose from a list. All coded
    needed at runtime to check if the extension is available and handle the
    data is provided by GDM as macros. This means that there are no
    additional run-time requirements.
    
    Related to https://pagure.io/SSSD/sssd/issue/3560
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Tested-by: Scott Poore <spoore@redhat.com>