A former Worcester attorney currently serving jail time for involuntary manslaughter is in trouble with the law again.   Federal prosecutors have filed firearms charges against 50-year-old David Manilla.   He shot and killed a Quakertown man while hunting more than two years ago.   Investigators say Manilla had more than 50 guns and a bag of ammunition removed from his home after the shooting incident.   He was barred from owning firearms of any kind following a felony conviction in 1985.