After 18 months of waiting, an average of 3.5 million people tuned in Sunday to watch “Mad Men’s” fifth-season debut. That’s a series best for the show — and about 600,000 more people than watched the fourth-season debut in July 2010. “Mad Men” had been off the air since October 2010, following the fourth-season finale, as the studio, the network and the creator wrangled over this and that — product placement within the show, adding another ad break, creator’s salary, blah, blah, blah.