Former FBI Agent R. Lindley DeVecchio fed a mobster-turned-informant confidential information that was used "to devastating effect,'' a prosecutor said Monday at his murder trial.Former Colombo crime family capo Gregory Scarpa brutally killed four people, prosecutor Joseph Alexis said, "and the defendant helped him do it.''But defense attorney Douglas Grover said prosecutors "are quick to provide broad strokes -- broad strokes that provide cover for witnesses who are telling lies.''DeVecchio went on trial Monday in state Supreme Court in Brooklyn in what prosecutors have billed as "one of the worst cases of law enforcement corruption in the history of this...