Now reading a source file and decoding it from UTF-16LE to UTF-8 is
done in a single function. Error messages are improved, and an error is
emitted when the source file has a BOM not supported (UTF-16BE, UTF-32).

Please note that the BOM of UTF-32 is composed of the same bytes as the
BOM of UTF-16 followed by a null character. Therefore a source file in
UTF-16LE starting with a null byte will be interpreted as an UTF-32, and
rejeted because of an invalid format. In pratice this is not a problem,
as the code would have been rejected later anyway because of the null
character.