Merge branch 'dt/http-range'

Merge branch 'dt/http-range'

A Range: request can be responded with a full response and when
asked properly libcurl knows how to strip the result down to the
requested range.  However, we were hand-crafting a range request
and it did not kick in.

* dt/http-range:
  http: use off_t to store partial file size
  http.c: use CURLOPT_RANGE for range requests