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.

bootstrap.conf

@@ -45,10 +45,14 @@ gnulib_modules="
   backupfile
   base32
   base64
-  btowc
+  btoc32
   buffer-lcm
   byteswap
   c-strcase
+  c32iscntrl
+  c32isprint
+  c32isspace
+  c32width
   canon-host
   canonicalize
   chmodat
@@ -161,6 +165,7 @@ gnulib_modules="
   malloc-gnu
   manywarnings
   mbrlen
+  mbrtoc32
   mbrtowc
   mbschr
   mbslen
@@ -282,7 +287,6 @@ gnulib_modules="
   verify
   verror
   version-etc-fsf
-  wcwidth
   winsz-ioctl
   winsz-termios
   write-any-file