It is as French as the croissant: The vast labyrinth of labor laws that for decades have ensured watertight job protections for people, no matter how poorly they perform, and handsome packages once they leave. But as the French get back to work this week after their long summer vacations (a minimum five weeks’ vacation is also enshrined in law), that labyrinth is crumbling, with the biggest overhaul in workplace rules in generations.