They called him “labor boy.”Marc J. Siegel’s labor and employment career began as a mediator at Duke University’s Durham Dispute Settlement Center, and it continued at the University of Illinois College of Law, where he took every labor and employment class available. It was at U. of I. that he received the nickname that would chart his career. “I enjoyed fighting for the underdog,” said Siegel, 43. “Maybe it was being a second child. I always wanted what I thought was fair …