spanking new CURLOPT_SEEKFUNCTION simply to take advantage of the improved
performance for the upload resume cases where you want to upload the last
few bytes of a very large file. To implement this decently, I had to switch
the client code for uploading from fopen()/fread() to plain open()/read() so
that we can use lseek() to do >32bit seeks (as fseek() doesn't allow that)
on systems that offer support for that.