request: eliminate request `getheader` bool, reverse `header` default

request: eliminate request `getheader` bool, reverse `header` default

Deduce that the transfer response expects headers by the protocol
handler implementing `write_resp_hd` callback. This eleminates the
`getheader` parameter in the `Curl_xfer_setup_*()` methods.

Add an implementation to RTSP for `write_resp_hd`, joining the HTTP
protocol in the only handlers having it.

Reverse the default of request's `header` bit that signals that headers
are expected. Default is now FALSE, set to TRUE when setting up the
transfer by presence of `write_resp_hd` in the protocol handler.

Closes #18218