By Kate Holton LONDON (Reuters) -When the BBC launched an expansion into the U.S. in June, its head of news promised "trust at a time of dramatic global uncertainty". Five months on, President Donald Trump is threatening a $5 billion lawsuit, governments long hostile to independent news are vowing to make life difficult for the British broadcaster and its news chief Deborah Turness has gone. The crisis has been sparked by the admission that in a piece that aired before last year's U.S. presidential election, the BBC's flagship documentary programme "Panorama" spliced together parts of Trump's speech on the day...