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-utf16.result

@@ -0,0 +1,13 @@
+#
+# MDEV-17088 Provide tools to encode/decode mysql-encoded file system names
+#
+SET NAMES utf8;
+# Bad delimiter
+--delimiter cannot be used with utf16 to utf8 conversion
+# Bad delimiter
+--delimiter cannot be used with utf8 to utf16 conversion
+# Start of file01.utf16.txt
+aaa
+xxxяяяxxx
+bbb
+# End of file01.utf16.txt