Place the neck and giblets, except for the liver (save for another use), in a large roasting pan. Set a roasting rack in the pan. Place the turkey on the rack breast-side up. Using your fingers, starting at the neck, loosen the skin from the breast, legs, and thighs. Tuck wings under the breast, if desired. Spread the remaining porcini butter under the skin of the turkey and all over the breast, legs, and thighs. Season the cavity and the outside of the turkey generously with salt and pepper. Stuff the cavity with the leek and rosemary sprigs. Brush the skin of the turkey with the oil. Pour 2 cups of water into the pan.