A one-of-its-kind flag that draped former President Abraham Lincoln’s casket during his funeral procession has found a new home in the Big Apple — at a Midtown steakhouse. Keens Steakhouse, the 141-year-old white tablecloth joint known for its extensive collection of Americana memorabilia, unveiled the gargantuan, half-million dollar auction find during a private unveiling ceremony Thursday. “It’s truly a treasure,” Keens Steakhouse general manager Julia Lisowski told The Post of the historic flag, which was used in procession after Lincoln was assassinated in office in 1865. “It’s a really special and amazing piece of history that we are so honored...