session: set reauth sessions as such
    
    Reauthentication sessions need subtly different behavior than initial
    login sessions with regard to pam.  For instance, we want to refresh
    existing kerberos credentials instead of establishing new kerberos
    credentials.
    
    GDM has all the code in place to do this properly, but lacks the actual
    call to mark reauthentication sessions as such.
    
    This commit adds the missing call.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=691269