Extend CURLFile to support streams

Extend CURLFile to support streams

Due to former restrictions of the libcurl API, curl multipart/formdata
file uploads supported only proper files.  However, as of curl 7.56.0
the new `curl_mime_*()` API is available (and already supported by
PHP[1]), which allows us to support arbitrary *seekable* streams, which
is generally desirable, and particularly resolves issues with the
transparent Unicode and long part support on Windows (see bug #77711).

Note that older curl versions are still supported, but CURLFile is
still restricted to proper files in this case.

[1] <http://git.php.net/?p=php-src.git;a=commit;h=a83b68ba56714bfa06737a61af795460caa4a105>