Not since Sheldon Cooper ran away has a television sitcom posted such high ratings. Some 21 years after its nine-season run on ABC, “Roseanne” returned to primetime on Tuesday in back-to-back episodes at 8 p.m. that averaged a 5.1 rating in adults 18-49 and more than 18 million total viewers. Those are huge numbers for a scripted show on broadcast television these days as digital video fractures the entertainment market.