In 1982, “Conan the Barbarian” enticed audiences with a poster promising four phases of a rough man’s rough life: “Thief. Warrior. Gladiator. King.” “The Northman,” which wanders narratively but, as cinema, basically eats “Conan” for breakfast, follows what might be considered a similar career path: Prince, followed by Slave, then Viking Marauder, and finally Newly Sensitized Lover and Potential Family Man. Alexander Skarsgard takes the title role, as well as taking a fair bit of on-screen punishment en route to a climactic battle at the Gates of Hel (one “l” in this hell, for the record). The…