Use default DBus prefix part III (server protocols)

This allows us to move to dbus_client_protos so our generated header gets added to generated_dbus_headers instead of generated_dbus_sources.

We keep the special handling of the protocol requiring an object manager to avoid a name class.

Marking as draft until !1805 (merged) lands but putting it out already so it is easier to test.

Closes: #1276 (closed) Signed-off-by: Guido Günther agx@sigxcpu.org