Compaq Computer chief executive Eckhard Pfeiffer, expecting basic shift in computer industry, is once again remaking world's top personal computer maker; seeks to fend off mounting threat from personal computer makers like Dell Computer and Gateway 2000 that are able to undercut Compaq and are growing faster by selling made-to-order PC's directly to customers; also seeks to make firm into full-fledged computer company, not just PC supplier; moves seem to herald another round of price cuts; graphs; photo (L)