df: avoid multibyte character corruption on macOS

* src/df.c (hide_problematic_chars): Use c_iscntrl() as
passing 8 bit characters to iscntrl() is not supported on macOS.
* NEWS: Mention the bug fix.
Fixes https://bugs.gnu.org/32236

NEWS

@@ -2,6 +2,10 @@ GNU coreutils NEWS                                    -*- outline -*-
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
+** Bug fixes
+
+  df no longer corrupts displayed multibyte characters on macOS.
+
 
 * Noteworthy changes in release 8.30 (2018-07-01) [stable]
 

src/df.c

@@ -23,6 +23,7 @@
 #include <sys/types.h>
 #include <getopt.h>
 #include <assert.h>
+#include <c-ctype.h>
 
 #include "system.h"
 #include "canonicalize.h"
@@ -281,7 +282,7 @@ hide_problematic_chars (char *cell)
   char *p = cell;
   while (*p)
     {
-      if (iscntrl (to_uchar (*p)))
+      if (c_iscntrl (to_uchar (*p)))
         *p = '?';
       p++;
     }