MEXICO CITY - Facing renewed U.S. allegations that theirs is a racist society, many Mexicans were scratching their heads again Thursday. The controversy surged Wednesday with news that Mexico is issuing postage stamps depicting Memin Pinguin, a rambunctious black youth who has been a popular comic-book character here for more than 60 years. Memin's physical attributes — exaggerated lips, large eyes and somewhat simian body language — might scream racism to sensitized minds. African-American leaders and U.S. government officials have denounced the stamp and Memin as relics of a bygone era when stereotypes were standard and racism was the rage....