NEW YORK — When Daniel Penny fatally choked a homeless man aboard a Manhattan subway last year, the 25-year-old veteran appeared to be using a combat technique that he learned in the U.S. Marines, according to the martial arts instructor who served alongside Penny and trained him in several chokeholds. But contrary to the training he received, Penny maintained his grip around the man’s neck after he seemed to lose consciousness, turning the non-lethal maneuver into a potentially deadly choke,