Disable resuming download of formatted document

Disable resuming download of formatted document

If the user chose File -> Save formatted document and typed the name
of an existing file, ELinks offered to resume downloading the file.
There are a few problems with that:

* save_formatted_finish does not actually support resuming.  It would
  instead overwrite the beginning of the file and not truncate it.

* When save_formatted calls create_download_file, cdf_hop->data
  ends up pointing to struct document.  If the user then chooses to
  resume, lun_resume would read *(int *)cdf_hop->data, hoping to
  get cmdw_hop.magic or codw_hop.magic.  struct document does not
  begin with any such magic value.

* Because ELinks already has the formatted document in memory,
  resuming saves neither time nor I/O.

So don't show the "Resume download of the original file" button in
this situation.