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.
The line endings broke when I saved the three recent patches (my fault,
not Colin's) to 'git am' them.
Adjusted the stripping of the test program for comparing to also exclude
the SSH key file name as that will differ and use a local path name.
