Insecure use of strcpy allowed for potential buffer overflow vulnerabili...

...ties.

Several of these changes have been replaced with the more secure strncpy. Due to the Contiki being the largest IoT platform, security should be a primary concerned.
This code introduces a memory overflow. You start writing at `webpage + some_offeset` but forget to take the offset into account by again telling the size to be `sizeof(webpage)`. So the correct statement would be

```C
strncpy(webpage + WWW_CONF_WEBPAGE_WIDTH * 5,
    (80 - WWW_CONF_WEBPAGE_WIDTH) / 2 + "\nThis web page cannot be displayed.",
    sizeof(webpage) - (WWW_CONF_WEBPAGE_WIDTH * 5));
```

Fix for the next line is similar to this one.