ATHENS (Reuters) - Greece has extended its offer to buy back debt until Tuesday, seeking more bids from bondholders after falling just short of a target to retire bonds worth 30 billion euros at a cost of just 10 billion euros. The buyback is designed to provide for about half of a 40-billion euro debt relief package for Athens agreed last month by the European Union and International Monetary Fund. Its success is crucial to ensuring Greece's debt is put back on sustainable footing and -- more immediately -- to unlocking badly-needed aid for the country. The offer had been due to end on ...