curl_easy_duphandle did not necessarily duplicate the CURLOPT_COOKIEFILE
option. It only enabled the cookie engine in the destination handle if
data->cookies is not NULL (where data is the source handle). In case of a
newly initialized handle which just had the cookie support enabled by a
curl_easy_setopt(handle, CURL_COOKIEFILE, "")-call, handle->cookies was
still NULL because the setopt-call only appends the value to
data->change.cookielist, hence duplicating this handle would not have the
cookie engine switched on.
We also concluded that the slist-functionality would be suitable for being
put in its own module rather than simply hanging out in lib/sendf.c so I
created lib/slist.[ch] for them.
