NEW YORK — Sagging, the practice of letting one’s pants hang slightly below the waist in a way that exposes the underwear, is a fashion trend that has inspired heated debate of a sort unseen since the mink stole, pitting supporters of free expression against those who see it as indecent exposure. Like fur, it likely will stick around for some time, so inventor Andrew Lewis has taken the pragmatic route with Subs, launched by Hatch Ventures.