REFACTOR: DFL_RSP_UMASK constant in responder code
    
    There is DFL_RSP_UMASK constant for very secure umask in responder
    code. This patch replaces occurances of value 0177 with this constant.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2424
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>