Add strnlen()

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@3054 42af7a65-404d-4744-a932-0658087f49c3

Documentation/NuttxUserGuide.html

@@ -6410,6 +6410,7 @@ char  *strchr(const char *s, int c);
 FAR char *strdup(const char *s);
 const char *strerror(int);
 size_t strlen(const char *);
+size_t strnlen(const char *, size_t);
 char  *strcat(char *, const char *);
 char  *strncat(char *, const char *, size_t);
 int    strcmp(const char *, const char *);