check validity of UTF-8 and ASCII files

 The utfcheck program examines a text file and prints a summary
 of what the file contains: ASCII, UTF-8, UTF-16 (either big-endian
 or little-endian based on an initial Byte Order Mark), or binary
 data. ASCII and UTF-8 files are processed further; UTF-16 and
 binary files are not. For a UTF-8 file, the summary includes
 whether or not the file begins with the Unicode Byte Order Mark
 (U+FEFF). Any following data encountered that is not well-formed
 ASCII or UTF-8 Unicode is considered to be binary data; upon
 reading such data the input file is considered not to be a proper
 text file and the program exits with an error status.
 .
 The utfcheck program returns an exit status of EXIT_SUCCESS if
 the text file was well-formed, and EXIT_FAILURE otherwise.