- Following r302512 (remove collation support for [a-z]-ranges in libc)
remove collation support for a-z ranges here too.
It was implemented for single byte locales only in any case.
- Reduce [Cc]flag loop to WCHAR_MAX, WINT_MAX here includes WEOF which is
not a character.
- Optimize [Cc]flag case: don't repeatedly add the last character of
string2 to squeeze cset when string2 reach its EOS state.
- Reflect in the manpage that [=equiv=] is implemented for single
byte locales only.