NATO SAYS SERBS, FEARING LAND WAR, DIG IN ON BORDER
Yugoslavia, anticipating that NATO may eventually mount ground offensive, has stationed army brigades along likely attack routes from Macedonia and Albania and is trying to fortify border; positioning of Serbian forces represents marked shift in Yugoslavia's military strategy from overseeing expulsion of ethnic Albanians from Kosovo to preparing for long battle, and focusing on blocking incursions by NATO troops as well as rebel fighters of Kosovo Liberation Army; it also signals that while two months of allied air strikes have weakened Serbian forces, it has not broken their will; map (M)