tuklib_mbstr_width: Change the behavior when wcwidth() is not available

tuklib_mbstr_width: Change the behavior when wcwidth() is not available

If wcwidth() isn't available (Windows), previously it was assumed
that one byte == one column in the terminal. Now it is assumed that
one multibyte character == one column. This works better with UTF-8.
Languages that only use single-width characters without any combining
characters should work correctly with this.

In xz, none of po/*.po contain combining characters and only ko.po,
zh_CN.po, and zh_TW.po contain fullwidth characters. Thus, "only"
those three translations in xz are broken on Windows with the
UTF-8 code page. Broken means that column headings in xz -lvv and
(only in the master branch) strings in --long-help are misaligned,
so it's not a huge problem. I don't know if those three languages
displayed perfectly before the UTF-8 change because I hadn't tested
translations with native Windows builds before.

Fixes: 46ee0061629fb075d61d83839e14dd193337af59