More than two-thirds of Massachusetts residents support the state's two-year-old, near-universal health insurance law, according to a new poll. The survey by the Harvard School of Public Health and the Blue Cross Blue Shield of Massachusetts Foundation found that 69% of those polled backed the law, an increase from the 61% who said they favored it when it was first adopted. Yet when pollsters asked the same question of residents who have bought health insurance or changed policies because of the law, support dropped to 52%.