NEW YORK (Reuters) - Stock index futures gained modestly Friday, with the retail sector in focus as the holiday shopping season begins, on a day of abbreviated day on Wall Street following the Thanksgiving holiday. Greece said the International Monetary Fund had relaxed its debt-cutting target for the country, suggesting lenders were closer to a deal for a vital aid tranche to be paid. But other sources involved in the talks cautioned the funding gap was far bigger than Greece has suggested. ...