Simon McVittie authored May 13, 2024 and Philip Withnall committed May 16, 2024

These well-known flags and replies are part of the D-Bus Specification,
and also exist with the same names in libdbus header files.
Moving them into a private header means that unit tests like
gdbus-proxy-threads and gdbus-subscribe don't have to reinvent them.

Signed-off-by: Simon McVittie <smcv@collabora.com>