- total_length() calls down the reader chain, delivers -1
  when length is not known
- encoding readers, which change length, will intercept this
  and return -1
- use `Curl_creader_total_length(data)` when evaluating
  rewind decisions

- the setup of a HTTP request is reshuffled to follow:
  * determine method, target, auth negotiation
  * install the client reader(s) for the request, including
    crlf conversions and "chunked" encoding
  * apply ranges to client reader
  * concat request headers, upgrades, cookies, etc.
  * complete request by determining Content-Length of
    installed readers in combination with method
  * send
- add methods for client readers to
  * return the overall length they will generate (or -1
    when that is not known)
  * return the amount of data on the CLIENT level, so that
    expect-100 can decide if it want to apply itself
  * set a "resume_from" offset or fail if unsupported
- struct HTTP has become largely empty now

- tell creader to rewind on next start
- Curl_client_reset() will keep reader for future rewind
  if requested
- add Curl_client_cleanup() for freeing all resources
  independent of rewinds
- add Curl_client_start() to trigger rewinds
- move rewind code from multi.c to sendf.c and make
  part of "cr-in"'s implementation

…used param

- when switching from POST to GET disable client rewinds as
  they will be no longer needed
- Client reader "cr_in": make a copy of fread_func and "in" data
  on creation and use those through the reader's lifetime.
  This allows a rewind to do the right thing on the old reader
  when a new request is starting, because data->state.fread_func
  is reset on entering the CONNECT state (very early).

…signed but never readfix ubuntu scanbuild, assigned but never readfix ubuntu scanbuild, assigned but never readfix ubuntu scanbuild, assigned but never readfix ubuntu scanbuild, assigned but never readfix ubuntu scanbuild, assigned but never readfix ubuntu scanbuild, assigned but never read

… hook.