Problem: The functions my_like_range_xxx() returned
badly formed maximum strings for Asian character sets,
which made problems for storage engines.

Fix: 
- Removed a number my_like_range_xxx() implementations,
  which were in fact dumplicate code pieces.
- Using generic my_like_range_mb() instead.
- Setting max_sort_char member properly for Asian character sets
- Adding unittest/strings/strings-t.c, 
  to test that my_like_range_xxx() return well-formed 
  min and max strings.

Notes:

- No additional tests in mysql/t/ available.
  Old tests cover the affected code well enough.