Larry King’s contract with CNN is set to expire in 18 months — and the cable network’s chiefs are busy lining up his successor in case he retires.King, 75, has been interviewing celebrities since 1985. Sources say CNN’s first choice to succeed him would be Ryan Seacrest — but his massive deal with “American Idol” makes him too expensive. Other contenders are CBS anchor Katie Couric, CNN “video wall guy” John King and Joy Behar, who is launching her own interview show on CNN’s Headline News.A sou