THIS promises to be the worst Labor Day in the memory of most Americans. Organized labor is down to about 7 percent of the private work force. Members of non-organized labor — most of the rest of us — are unemployed, underemployed or underwater. The Labor Department reported on Friday that just 67,000 new private-sector jobs were created in August, while at least 125,000 are needed to keep up with the growth of the potential work force.