Detectives last night nabbed a man who has been using the cell phone of Martín Barreto, the former Giuliani administration press aide found strangled in a Greenwich Village apartment last week, police sources said. Police stopped short of calling the unidentified man a suspect in the murder of Barreto, who was found naked on his bed Aug. 21. But they were questioning the man last night at the 6th Precinct and had not ruled him out as the killer. Sources said detectives were eager to check his DNA against evidence from the crime scene. Investigators believe Barreto, 48, a childhood...