Raging floods and mudslides have killed at least 57 people in southern Brazil and forced nearly 70,000 to flee their homes.   The country’s civil defense agency also confirmed that at least 74 people were injured and another 67 missing from the catastrophic flooding.   The toll did not include two people who died in an explosion at a flooded gas station in Porto Alegre, where rescue crews were attempting to refuel.   Fast-rising water levels in the state of Rio Grande do Sul were straining dams and particularly threatening economically important Porto Alegre, a city of 1.4 million.   The Guaiba River, which flows through the city, is at a historic high of 5.04 meters (16.5 feet), well above the 4.76 meters that had stood as a record since devastating 1941 floods.   Authorities scrambled to evacuate swamped neighborhoods as rescue workers used four-wheel-drive vehicles — and even jet skis — to maneuver through waist-deep water in search of the stranded.