Carnival Cruise Lines canceled voyages through April aboard the Triumph, a ship plagued by mechanical problems in recent weeks and finally­ left powerless in the Gulf of Mexico after an engine-room fire on Sunday. Carnival’s announcement came Wednesday as the Triumph­ was being towed to Mobile, Ala., with more than 4,000 people on board, some of whom have told relatives they have limited food and access­ to bathrooms.