cli tool: reimplement stdin buffering in -F option.

If stdin is not a regular file, its content is memory-buffered to enable
a possible data "rewind".
In all cases, stdin data size is determined before real use to avoid
having an unknown part's size.

--libcurl generated code is left as an unbuffered stdin fread/fseek callback
part with unknown data size.

Buffering is not supported in deprecated curl_formadd() API.