Make struct tevent_req opaque

Make struct tevent_req opaque

Move struct tevent_req in tevent_internal, and ad getters and setters
for private data and the callback function.
This patch also renames 'private_state' into 'data'. What is held in this
pointer is in fact data and not a state like enum tevent_req_state.
Calling it 'state' is confusing.

The functions addedd are:
tevent_req_set_callback() - sets req->async.fn and req->async.private_data
tevent_req_set_print_fn() - sets req->private_print
tevent_req_callback_data() - gets req->async.private_data
tevent_req_data() - gets rea->data

This way it is much simpler to keep API/ABI compatibility in the future.