By Ulf Laessing and Asma Alsharif CAIRO (Reuters) - Egypt's central bank sold $600 million to banks in a special auction of foreign exchange on Sunday to pay for wheat, meat, cooking oil and other essential imports to a country struggling with a currency crisis. The size of the auction - 15 times the amount the central bank has been selling at its regular currency auctions - showed the extent of pent up demand for dollars as Egypt struggles with an economic crisis two years after the ousting of Hosni Mubarak. ...