printf: quote the ignored characters in the warning

* src/printf.c (STRTOX): Add a call to quote when printing the ignored
characters.
* tests/printf/printf.sh: Adjust an existing test case. Add another
quoting test case.

Discovered by Anthropic's OSS Scanner, as bug ANT-2026-I9I24ON7.

src/printf.c

@@ -178,7 +178,7 @@ FUNC_NAME (char const *s)						 \
          printf is ignoring those characters.  If POSIXLY_CORRECT *is*	 \
          set, then don't give the warning.  */				 \
       if (*++s != 0 && !posixly_correct)				 \
-        error (0, 0, _(cfcc_msg), s);					 \
+        error (0, 0, _(cfcc_msg), quote (s));				 \
     }									 \
   else									 \
     {									 \

tests/printf/printf.sh

@@ -125,8 +125,9 @@ EOF
 # Map that output to the more common expected output.
 sed 's/: Invalid.*/: expected a numeric value/' err > k && mv k err
 
+ignore='character(s) following character constant have been ignored'
 cat <<EOF > exp_err
-printf: warning: ": character(s) following character constant have been ignored
+printf: warning: '"': $ignore
 printf: '"': expected a numeric value
 printf: 'a': expected a numeric value
 EOF
@@ -134,6 +135,14 @@ EOF
 compare exp out || fail=1
 compare exp_err err || fail=1
 
+NL='
+'
+echo 97 > exp || framework_failure_
+echo "printf: warning: 'b\nc': $ignore" >exp-err || framework_failure_
+$prog '%d\n' '"ab'"$NL"'c' >out 2>err || fail=1
+compare exp out || fail=1
+compare exp-err err || fail=1
+
 # Ensure large field widths and precisions produce fully padded output
 $prog '%70000d' 1 > out || fail=1
 test "$(wc -c < out)" = 70000 || fail=1