To achieve this, first new structure HeaderData is defined to hold
necessary data to perform header-related work. Then tool_header_cb now
receives HeaderData pointer as userdata. All header-related work
(currently, dumping header and Content-Disposition inspection) are done
in this callback function. HeaderData.outs->config is used to determine
whether each work is done.
Unit tests were also updated because after this change, curl code always
sets CURLOPT_HEADERFUNCTION and CURLOPT_HEADERDATA.
Tested with -O -J -D, -O -J -i and -O -J -D -i and all worked fine.
