http: with chunked POST forced, disable length check on read callback

http: with chunked POST forced, disable length check on read callback

- when an application forces HTTP/1.1 chunked transfer encoding
  by setting the corresponding header and instructs curl to use
  the CURLOPT_READFUNCTION, disregard any POST length information.
- this establishes backward compatibility with previous curl versions

Applications are encouraged to not force "chunked", but rather
set length information for a POST. By setting -1, curl will
auto-select chunked on HTTP/1.1 and work properly on other HTTP
versions.

Reported-by: Jeff King
Fixes #13229
Closes #13257