A cleanup for MDEV-17088 Provide tools to encode/decode mysql-encoded file system names

- 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

mysql-test/suite/client/mariadb-conv-utf8.result

@@ -21,6 +21,8 @@ BINARY CONVERT(a USING filename)
 測試資料2
 test/測試資料.frm
 test/測試資料2.frm
+test/測試資料.frm
+test/測試資料2.frm
 DROP TABLE t1;
 # bulk convert with file
 # --- Start of mariadb-conv for mysql-conv-test-utf8.txt ---