A transfer may do several SingleRequests for its success. This happens regularly for authentication, follows and retries on failed connections. The "readwrite()" calls and functions connected to those carried a bool *done parameter to indicate that the current SingleRequest is over. This may happen before upload_done or download_done bits of SingleRequest are set.

The problem with that is now write_resp() protocol handlers are invoked in places where the bool *done cannot be passed up to the caller. Instead of being a bool in the call chain, it needs to become a member of SingleRequest, reflecting its state.

This PR removes the bool *done parameter and adds the done bit to SingleRequest instead. It adds Curl_req_soft_reset() for using a SingleRequest in a follow up, clearing done and other flags/counters.