You know you are in for a more unsettling type of comedy when the plot's kickoff is an AIDS-afflicted heroin addict jabbing a 7-year-old girl with a contaminated needle. Even more unsettling is that you find yourself laughing at this. But such is the infectious way of Jamie Linley's Dirty Works, Stiff Upper Lip's sophomore effort now playing at the Greenwich Street Theatre. In a kind of British answer to Trainspotting, Linley takes us to the heart of a London slum and a small crew of nobodies eking out their short lives through a haze of petty crime, promiscuity, and all manner of intoxicants