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.
curl_mime_encoder() is operational and documented.
curl tool -F option is extended with ";encoder=".
curl tool --libcurl option generates calls to curl_mime_encoder().
New encoder tests 648 & 649.
Test 1404 extended with an encoder specification.
Patrick Monnerat