A relentless bull-run in U.S. stock indexes this year has primed the pump for a strong IPO market in 2018, with several multibillion-dollar corporations including Airbnb and Spotify widely expected to grab headlines with their offerings.The S&P 500, the Dow Jones Industrial Average and the Nasdaq are all set to cap the year with their biggest annual gains since 2013.Riding the rally, the U.S.