The program converts a GB file to a BIG-5 file, or a BIG-5 file to a GB file.
GB (GuoBiao) refers to the standard implementation of "GB2312-80" of
Mainland China, in which the two bytes representing a GB code have their
most significant bit set to 1.  BIG-5 refers to the Big Five standard
published in 1984 by Taiwan's Institute for Information Industry.
