MOGADISHU, Somalia — Two decades after dead American soldiers were dragged through the streets of Mogadishu, John Kerry on Tuesday became the first secretary of state to set foot in Somalia, a symbolic visit to show support for the African nation’s fledgling government and the United States’ readiness to move past a dark chapter in its history. [...] as Kerry vowed to deepen America’s partnership with a new cadre of Somali leaders, the fact that he never left the airport underscored just how dangerous and unstable Somalia remains after a quarter-century of civil war. Before returning to Kenya, the top U.S.