They were three hipper-than-hip undercover cops with a touch of menace and plenty of attitude. They walked the walk and talked the talk--and an entire generation stepped into line. Julie, Linc and Pete--Peggy Lipton, Clarence Williams III and Michael Cole, respectively--changed the television landscape. Now, take a journey back to the turbulent '60s and relive the phenomenon known as The Mod Squad, a bold TV series that redefined fashion, from hairstyles to language.   "Solid." One Black, One White, One Blonde.................. The early cop show format, as defined by shows like Dragnet, was certainly entertaining, but let's face it¦sometimes those flatfoots could be a bit square. The cops doled out justice, but they didn't have a lot of fun doing it. It seemed that the description of TV cops might never include the word "cool," until (whew!) The Mod Squad showed up to save the day.