Saving some cpu cycles in http response header processing:
- pass the length of the header line along
- use string constant sizeof() instead of strlen()
- check line length if prefix is possible
- switch on first header char to limit checks

- try send the complete request directly without copying into
  the send buf first. Buffer the remained that could not be sent
  in on go.

- initial sends contains also header bytes that should not
  be counted against blocked length
- fixes (hopefully) assertion failure in fuzzing