UTF-8 -> CP437 translation support for files (with .utf8 extension or beginning

with ZWNBSP/BOM UTF-8 sequence) and messages (with the "CHRS: UTF-8"
FTN control paragra or MIME "charset=utf-8" parts).
Not all UNICODE codepoints are supported (obviously).
Tested with various files from ftp://columbia.edu/kermit/charsets/
(e.g. test.utf8, utf8-boxes.txt)