Floyd M. Boring, a Secret Service agent who guarded five presidents and took part in the gunfight that foiled an attempt by Puerto Rican nationalists to assassinate President Harry S. Truman, died Friday at his home in Silver Spring, Md. He was 92. His death was confirmed by the Collins Funeral Home of Silver Spring. On the afternoon of Nov. 1, 1950, Truman was taking a nap at Blair House, where he was living while the White House, across Pennsylvania Avenue, was being renovated. Boring was stationed outside Blair House with several uniformed White House guards, while two Secret Service...