- Brian Ulm figured out that if you did an SFTP upload with

  CURLOPT_FTP_CREATE_MISSING_DIRS to create a directory, and then re-used the
  handle and uploaded another file to another directory that needed to be
  created, the second upload would fail. Another case of a state variable that
  wasn't properly reset between requests.

- I rewrote the 100-continue code to use a single state variable instead of
  the previous two ones. I think it made the logic somewhat clearer.