Set MB_LEN_MAX to value that equals UTFmax in case some APE program uses it.

Reference: /n/atom/patch/applied2013/ape-limits
Date: Thu Aug 22 23:53:12 CES 2013
Signed-off-by: jas@corpus-callosum.com

--- /sys/include/ape/limits.h	Thu Aug 22 23:45:53 2013
+++ /sys/include/ape/limits.h	Thu Aug 22 23:45:54 2013
@@ -3,7 +3,7 @@
 /* 8 bit chars (signed), 16 bit shorts, 32 bit ints/longs */
 
 #define CHAR_BIT	8
-#define MB_LEN_MAX	3
+#define MB_LEN_MAX	4
 
 #define UCHAR_MAX	0xff
 #define USHRT_MAX	0xffff