lib: move 'done' parameter to SingleRequests

lib: move 'done' parameter to SingleRequests

A transfer may do several `SingleRequest`s 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 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.

Closes #13096