transfer: readwrite improvements

transfer: readwrite improvements

- changed header/chunk/handler->readwrite prototypes to accept `buf`,
  `blen` and a `pconsumed` pointer. They now get the buffer to work on
  and report back how many bytes they consumed
- eliminated `k->str` in SingleRequest
- improved excess data handling to properly calculate with any body data
  left in the headerb buffer
- eliminated `k->badheader` enum to only be a bool

Closes #12283