PYEONGCHANG, South Korea — Signs posted around the Olympic venues urge extreme caution. Nine hundred troops stream into the area to help. Worried organizers sequester 1,200 people in their rooms. Officials are scrambling on the eve of the biggest planned event in South Korea in years — not because of anything related to North Korea and national security, but in an attempt to arrest the spread of norovirus at the Pyeongchang Games. Media are fretting over a “virus panic.” South Koreans, typically quick to air their views online, have poured scorn on the government’s response and preparations.