France was on edge Saturday on the eve of its most unpredictable presidential election in decades, which will take place under heightened security after the jihadist killing of a policeman. The shooting on Paris's world-renowned Champs Elysees avenue on Thursday, claimed by the Islamic State group, thrust questions of security to the fore of campaigning after nine months of relative calm. On Saturday, police arrested a man carrying a knife at Paris's Gare du Nord station, briefly causing panic as some passengers rushed out of the way. France goes to the polls on Sunday in an election seen as crucial for the future of a deeply divided country and the beleaguered European Union. Surveys suggest that nearly a quarter of voters are still undecided and that until now the French have been more concerned…