While so many of his country contemporaries rush headlong into the middle of the road, Alan Jackson still keeps it lean and pure. On his fifth album Everything I Love, he transforms ''Who's Cheatin' Who,'' Charly McClain's fluffy country-pop hit, into muscular 'tonk and makes ''Between the Devil and Me,'' Harley Allen's dramatic ballad of sexual temptation and eternal damnation, throb with anxiety. A-

