NEW YORK, April 20 : SpaceX plans to cement founder Elon Musk's control after its IPO, granting him and a small group of insiders super-voting shares that will outweigh other investors, according to excerpts of the company's IPO filing reviewed by Reuters. The prospectus, which was confidentially filed this month, provides fresh details of the company’s financials and corporate governance. Upon completion of the offering, Musk will stay on as chief executive officer, chief technical officer,