NAIROBI, Kenya — A severe food shortage has deteriorated into a famine in two counties in South Sudan, the government and United Nations announced Monday, with 100,000 people facing starvation. Joyce Luma, head of the World Food Program in South Sudan, called the famine “man-made,” blaming it on political turmoil in a country engulfed in civil war since late 2013. Luma and representatives of two other U.N.