The two San Francisco police officers who shot to death a 45-year-old homeless man who allegedly lunged at one of them with a large kitchen knife in the Mission District acted lawfully and with legitimate concern for their lives, the city attorney’s office said. Responding to a civil rights lawsuit filed by the family of Luis Gongora — the homeless man who was shot to death near a homeless encampment in the 400 block of Shotwell Street in April — lawyers for the city dismissed eyewitness accounts as not credible in justifying the actions of the officers.