The May sweeps are upon us, which means a steady stream of movies with the word murder in the title. So the battle for ratings goes on, even though the war is over. The 1992-93 Nielsen TV season officially ended last week with CBS winning for the second year in a row, ABC finishing a respectable second and NBC, which a short time ago seemed invincible, limping along in third, several lengths behind. Fox, banging its drum of excess and experimentation, loudly brought up the rear. Someday Fox’s turn will come. In the cyclical TV ratings war, it is ever true that the last will be first. CBS dominated the ’60s, ABC the ’70s, NBC the ’80s. CBS has the jump on the ’90s, but it’s early yet, and the playing field is shifting all the time.