“Tragedy is when I cut my finger,’’ Mel Brooks once said. “Comedy is when you fall into an open sewer and die.’’ What is it, then when, you’re kidnapped and shipped to a legal black site and left to rot for half a decade? This is the question posed by Alex Gilvarry’s lively debut novel, “From the Memoirs of a Non-Enemy Combatant’’ a book that begins as a comedy, turns into a madcap, and then ends with a sober political backdraft.