Updating libutf to a newer version.

This brings us in sync with the google3 //third_party/utf
implementation.

Change-Id: I0f3e013304bab6d609ddf0b2619f3b5b8ec6f047
diff --git a/utfnlen.c b/utfnlen.c
index 6680329..d673c82 100644
--- a/utfnlen.c
+++ b/utfnlen.c
@@ -7,22 +7,22 @@
  * or modification of this software and in all copies of the supporting
  * documentation for such software.
  * THIS SOFTWARE IS BEING PROVIDED "AS IS", WITHOUT ANY EXPRESS OR IMPLIED
- * WARRANTY.  IN PARTICULAR, NEITHER THE AUTHORS NOR LUCENT TECHNOLOGIES MAKE
- * ANY REPRESENTATION OR WARRANTY OF ANY KIND CONCERNING THE MERCHANTABILITY
+ * WARRANTY.  IN PARTICULAR, NEITHER THE AUTHORS NOR LUCENT TECHNOLOGIES MAKE ANY
+ * REPRESENTATION OR WARRANTY OF ANY KIND CONCERNING THE MERCHANTABILITY
  * OF THIS SOFTWARE OR ITS FITNESS FOR ANY PARTICULAR PURPOSE.
  */
 #include <stdarg.h>
 #include <string.h>
-#include "plan9.h"
 #include "utf.h"
+#include "utfdef.h"
 
 int
-utfnlen(char *s, long m)
+utfnlen(const char *s, long m)
 {
 	int c;
 	long n;
 	Rune rune;
-	char *es;
+	const char *es;
 
 	es = s + m;
 	for(n = 0; s < es; n++) {