Add compat sbin Provides
    
    In preparation for https://fedoraproject.org/wiki/Changes/Unify_bin_and_sbin.
    About a dozen packages require /usr/sbin/sendmail.
    php-phpmailer6 has BuildRequires: /usr/sbin/smtp-sink.
    
    Also, use macro instead of hardcoding "sbindir", so that it becomes
    /usr/bin automatically when the macro definition is updated.