Implement strndup(3) using strnlen(3).

This makes the implementation a bit more consistent with strdup(3),
which uses strlen(3).

lib/libc/string/strndup.c

@@ -42,9 +42,7 @@ strndup(const char *str, size_t n)
 	size_t len;
 	char *copy;
 
-	for (len = 0; len < n && str[len]; len++)
-		continue;
-
+	len = strnlen(str, n);
 	if ((copy = malloc(len + 1)) == NULL)
 		return (NULL);
 	memcpy(copy, str, len);