CBS Aims At 10 P.M. To Help Letterman
CBS is challenging NBC's dominance of 10 pm slot with its strongest slate of programs in years; is also trying to raise quality of its stations' 11 pm newscasts will make them more competitive with NBC's; moves are aimed at boosting David Letterman's show in its competition with Jay Leno's Tonight show; ABC used Letterman's disgruntlement over weak lead-ins in failed bid to woo him away from CBS; table of lead-in shows; photos (M)