maint: prefer char32_t to wchar_t

maint: prefer char32_t to wchar_t

This should work better on non-glibc platforms that don’t
use Unicode for wchar_t.  However, POSIX appears to prohibit
this for printf.c so leave that alone.
* bootstrap.conf (gnulib_modules): Add btoc32, c32iscntrl,
c32isprint, c32isspace, c32width, mbrtoc32.  Remove btoc, wcwidth.
* src/df.c, src/ls.c, src/wc.c:
Include uchar.h instead of wchar.h and wctype.h.
* src/df.c (replace_invalid_chars):
* src/ls.c (quote_name_buf):
* src/wc.c (isnbspace, wc):
Use char32_t instead of wchar_t.