SERVER-6086 linenoise handles backspace incorrectly with Korean character set

 - use mk_wcswidth before refresh line. for calculate width.
You can ignore the earlier suggestion on static_cast. That's not correct. It should be fine to use reinterpret_cast to convert from UChar32* to const int* for mk_wcswidth without the intermediate void*.

Also, mk_wcswidth returns -1 on error. In that case, would it make sense to return "n" to fall back on previous behavior or pass along -1 to calculateScreenPosition?