The legacy of William Friedkin ’s 1980 erotic thriller “ Cruising ” is a complex one. A film long vehemently denounced by the queer community it purported to represent, and more recently reclaimed as a rare mainstream portrait of a vanished social scene, it is many things to many people — and in disentangling its onscreen achievements and errors in judgment from its heated production history, “ Mineshaft: The Cruising Murders ” already has a lot to do. But Jeffrey Schwarz ’s engaging,