Fix #79019: Copied cURL handles upload empty file

To cater to `curl_copy_handle()` of cURL handles with attached
`CURLFile`s, we must not attach the opened stream, because the stream

Fix #79019: Copied cURL handles upload empty file

To cater to `curl_copy_handle()` of cURL handles with attached
`CURLFile`s, we must not attach the opened stream, because the stream
may not be seekable, so that we could rewind, when the same stream is
going to be uploaded multiple times. Instead, we're opening the stream
lazily in the read callback.

Since `curl_multi_perfom()` processes easy handles asynchronously, we
have no control of the operation sequence. Since duplicated cURL
handles may be used with multi handles, we cannot use a single arg
structure, but actually have to rebuild the whole mime structure on
handle duplication and attach this to the new handle.

In order to better test this behavior, we extend the test responder to
print the size of the upload, and patch the existing tests accordingly.

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