bus-proxyd: assorted cleanups and fixes Just some cleanups around policy checks that came up during review. The code is still not productive.

bus-policy: split API for bus-proxyd Instead of operating on an sd_bus_message object, expose an API that has 4 functions: policy_check_own() policy_check_hello() policy_check_recv() policy_check_send() This also allows dropping extra code to parse message contents - the bus proxy already has dedicated code paths for that, and we can hook into those later. Tests amended accordingly.

bus-policy: add test utility Add some test files and routines for dbus policy checking.