Mention that you’re cruising the Nile and you’ll hear it: “Like ‘Death on the Nile’?” Agatha Christie’s 1937 whodunnit made us imagine Egypt aboard a chintzy cruise with cream teas. That’s a relic. Today there’s a better, more authentic way to see the ancient tombs and palaces along the Nile’s shores. Just step aboard a dahabiya. They’re strange looking shallow sailing vessels, with two big masts at each end that date back to the pharaohs.…