This patch changes the main name of 3 byte character set from utf8 to
utf8mb3. New old_mode UTF8_IS_UTF8MB3 is added and set TRUE by default,
so that utf8 would mean utf8mb3. If not set, utf8 would mean utf8mb4.
- Load and convert the entire input file at once,
rather than reading string-by-string using fgets().
This change makes it possible to convert from UCS2, UTF16, UTF32 data.
- Adding the --delimiter command, to treat the specified
characters as delimiters rather than data to convert.
Useful in combination with `-f filename` or `-t filename`.
The delimiter characters are not converted,
they are copied from the input to the output as is.
- Adding diagnostics with line number and position if:
* an illegal input byte sequence was found
* a character cannot be converted to the target character set
