TRENTON — Nondescript minivans pulled up like clockwork outside Newark’s homeless shelters, and the drivers offered a deal few desperate people would turn down: a free health checkup and a guaranteed prescription for painkillers, which can fetch as much as $100 per pill on the street. They were bused 10 miles away, authorities said, to a rundown strip mall in Passaic where the deal went down. A doctor offered a quick, often bogus exam before writing scripts for OxyContin or oxycodone. As a special thank you, he threw in a $10 gift card. His patients could take the drugs or...