THE F-B-I IS TRYING TO STEM THE FLOOD OF FORGED E-MAIL MESSAGES SENT TO MEMBERS OF CONGRESS. THE MESSAGES THREATEN TO DELETE EVERY FILE ON COMPUTERS ON CAPITOL HILL. THE "SAN JOSE MERCURY NEWS" SAYS THE MESSAGES CLAIM TO BE FROM A "GANG OF CYPHERPUNKS" DEDICATED TO WIPING OUT THE COMPUTER SYSTEMS OF THE NATION'S LAWMAKERS. THE E-MAIL MESSAGES CARRY THE RETURN ADDRESSES OF SOME LEGITIMATE INTERNET USERS. BUT THE IDEN