interface problems:

  o with pipelining disabled, the state should never be set to WAITDO but
    rather go straight to DO

  o we had multiple states for which the internal function returned no socket
    at all to wait for, with the effect that libcurl calls the socket callback
    (when curl_multi_socket() is used) with REMOVE prematurely (as it would be
    added again within very shortly)

  o when in DO and DOING states, the HTTP and HTTPS protocol handler functions
    didn't return that the socket should be waited for writing, but instead it
    was treated as if no socket was needing monitoring so again REMOVE was
    called prematurely.