bus-policy: split API for bus-proxyd

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.