1. PUT/POST without a known auth to use (possibly no auth required):

   (When explicitly set to use a multi-pass auth when doing a POST/PUT,
   libcurl should immediately go the Content-Length: 0 bytes route to avoid
   the first send all data phase, step 2. If told to use a single-pass auth,
   goto step 3.)

   Issue the proper PUT/POST request immediately, with the correct
   Content-Length and Expect: headers.

   If a 100 response is received or the wait for one times out, start sending
   the request-body.

   If a 401 (or 407 when talking through a proxy) is received, then:

   If we have "more than just a little" data left to send, close the
   connection. Exactly what "more than just a little" means will have to be
   determined. Possibly the current transfer speed should be taken into
   account as well.

   NOTE: if the size of the POST data is less than MAX_INITIAL_POST_SIZE (when
   CURLOPT_POSTFIELDS is used), libcurl will send everything in one single
   write() (all request-headers and request-body) and thus it will
   unconditionally send the full post data here.

2. PUT/POST with multi-pass auth but not yet completely negotiated:

   Send a PUT/POST request, we know that it will be rejected and thus we claim
   Content-Length zero to avoid having to send the request-body. (This seems
   to be what IE does.)

3. PUT/POST as the last step in the auth negotiation, that is when we have
   what we believe is a completed negotiation:

   Send a full and proper PUT/POST request (again) with the proper
   Content-Length and a following request-body.

   NOTE: this may very well be the second (or even third) time the whole or at
   least parts of the request body is sent to the server. Since the data may
   be provided to libcurl with a callback, we need a way to tell the app that
   the upload is to be restarted so that the callback will provide data from
   the start again.  This requires an API method/mechanism that libcurl
   doesn't have today. See below.

Data Rewind

   It will be troublesome for some apps to deal with a rewind like this in all
   circumstances. I'm thinking for example when using 'curl' to upload data
   from stdin. If libcurl ends up having to rewind the reading for a request
   to succeed, of course a lack of this callback or if it returns failure, will
   cause the request to fail completely.

   The new callback is set with CURLOPT_IOCTLFUNCTION (in an attempt to add a
   more generic function that might be used for other IO-related controls in
   the future):

     curlioerr curl_ioctl(CURL *handle, curliocmd cmd, void *clientp);

   And in the case where the read is to be rewinded, it would be called with a
   cmd named CURLIOCMD_RESTARTREAD. The callback would then return CURLIOE_OK,
   if things are fine, or CURLIOE_FAILRESTART if not.

Backwards Compatibility

   The approach used until now, that issues a HEAD on the given URL to trigger
   the auth negotiation could still be supported and encouraged, but it would
   be up to the app to first fetch a URL with GET/HEAD to negotiate on, since
   then a following PUT/POST wouldn't need to negotiate authentication and
   thus avoid double-sending data.

   Optionally, we keep the current approach if some option is set
   (CURLOPT_HEADBEFOREAUTH or similar), since it seems to work fairly well for
   POST on most servers.