-j: Inhibit multiple includes

File inclusion via FWEB's @i command suffers from a design deficiency: they cannot be inhibited by means of FWEB's preprocessor commands. (The reason is that @i is processed very early in the input stage, before tokenization. This design decision was inherited from CWEB, and is very difficult to change.) A particularly annoying situation arises when the same file is included multiple times; various array space may be eaten up unnecessarily. The -j option inhibits such multiple includes.