MOMBASA, Kenya, April 11 (Reuters) - A U.S.-flagged ship that was seized by Somali pirates arrived safely in the Kenyan port of Mombasa on Saturday, as a Somali mediator headed to sea to try to secure the release of the ship's American captain. "The captain is a hero," one crew member shouted from the 17,000-ton Maersk Alabama container ship as it docked. "He saved our lives by giving himself up." The ship, under the command of Richard Phillips, was attacked by gunmen far out in the Indian Ocean on Wednesday but its 20 American crew apparently fought off the hijackers...