Republican presidential nominee Donald Trump said in an interview here that he remains unwilling to say that President Obama is born in the United States, that he is more bullish than ever on his chances to win and that he is not exploring the launch of a new media company in case he loses the race. Trump also made a far-from-subtle push — in the interview and in a letter from his doctor released Thursday — to be seen as vigorous and healthy as his Democratic rival, Hillary Clinton, returned to the campaign trail after being treated for mild pneumonia.