Murder charges have been filed against an Old Bridge man accused of fatally stabbing an acquaintance.  Robert McGranahan also faces a weapons charge in the death of 65-year-old Edward Demko Jr., whose body was found early Saturday at his Sayreville home.  The 26-year-old McGranahan was arrested shortly after Demko's body was found by police.  Middlesex County prosecutors have not disclosed a possible motive for the slaying, but they don't believe the stabbing was a random act.  They say McGranahan and Demko knew each other, but further details on their relationship were not disclosed.  McGranahan remained jailed Sunday, and it was not known if he has retained a lawyer.