By James Mackenzie ZAPORIZHZHIA, Ukraine (Reuters) – As Dmytro Kartavov and his family joined thousands of people trying to flee the bombed-out city of Mariupol, one additional obstacle awaited as Russian troops sought to identify anyone fighting with Ukrainian forces defending the city. “They stripped the men naked, looked for tattoos,” said Kartayov, a 32 year-old builder, who said the troops paid particular attention to men’s knees. “I work, I do repairs, naturally my knees – these are working knees. They say – (you) climbed trenches, dug, and the like.” Speaking in a supermarket that has b…