- Linus Nielsen Feltzing reported and helped me repeat and fix a problem with

- Linus Nielsen Feltzing reported and helped me repeat and fix a problem with
  FTP with the multi interface: when a transfer fails, like when aborted by a
  write callback, the control connection was wrongly closed and thus not
  re-used properly.

  This change is also an attempt to cleanup the code somewhat in this area, as
  now the FTP code attempts to keep (better) track on pending responses
  necessary to get read in ftp_done().