The unhinged man who allegedly knifed a stranger in the back in a Lower Manhattan subway station last week was busted Wednesday, authorities said.  Dariel Castillo, 28 – who has four prior arrests – was charged with assault for allegedly stabbing a 51-year-old man in the Broadway-Lafayette hub around 5:10 a.m. Jan. 28, cops said.  Castillo allegedly started a fight with the victim for unknown reasons on the steps of the station before turning violent — repeatedly plunging the blade into his