Split reply header and payload buffers. Making it more coherent with all the
other request handlers, and allows a lot of further simplification by using
generic macros (coming in subsequent commits).
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1600>