By Trevor Hunnicutt, Ariba Shahid and Muhammad Al GebalyWASHINGTON/ISLAMABAD, April 19 - Iran's top negotiator said recent talks with the U.S. had made progress but gaps remained over nuclear issues and theStrait of Hormuz, while President Donald Trump cited "very good conversations" with Tehran despite warning against "blackmail" over the key shipping channel.Neither side offered any specifics ab