As the Competition Intensifies, HBO Chief Has Impressive Start
Jeffrey Bewkes, chairman and president of Home Box Office, hopes to keep leading cable television channel on top in increasingly competitive field; his predecessor, Michael Fuchs, is credited with building HBO into most popular premium movie service by making savvy deals for films from major studios and choosing successful original programming; Bewkes is continuing that strategy, but he faces stiffer competition as many more premium channels and advertiser-supported basic cable channels compete for viewers; photo; chart (L)