- Hans-Jurgen May pointed out that trying SCP or SFTP over a SOCKS proxy

  crashed libcurl. This is now addressed by making sure we use "plain send"
  internally when doing the socks handshake instead of the Curl_write()
  function which is designed to use the "target" protocol. That's then SCP or
  SFTP in this case. I also took the opportunity and cleaned up some ssh-
  related #ifdefs in the code for readability.