`Curl_hyper_stream` needs to distinguish between two kinds of
`HYPER_TASK_EMPTY` tasks: (a) the `foreach` tasks it creates itself, and
(b) background tasks that hyper produces. It does this by recording the
address of any `foreach` task in `hyptransfer->endtask` before pushing
it into the executor, and then comparing that against the address of
tasks later polled out of the executor.

This works right now, but there is no guarantee from hyper that the
addresses are stable. `hyper_executor_push` says "The executor takes
ownership of the task, which should not be accessed again unless
returned back to the user with `hyper_executor_poll`". That wording is a
bit ambiguous but with my Rust programmer's hat on I read it as meaning
the task returned with `hyper_executor_poll` may be conceptually the
same as a task that was pushed, but that there are no other guarantees
and comparing addresses is a bad idea.

This commit instead uses `hyper_task_set_userdata` to mark the `foreach`
task with a `USERDATA_RESP_BODY` value which can then be checked for,
removing the need for `hyptransfer->endtask`. This makes the code look
more like that hyper C API examples, which use userdata for every task
and never look at task addresses.

Closes curl#11779