Rewrite dbus proxy code

* Fix readiness signaled before the proxy is actually ready. The new
  code uses the --fd option of xdg-dbus-proxy to figure out when the
  proxy is ready. This might fix #18485.
* Run xdg-dbus-proxy in a daemonized process and exit the Python script
  when the proxy is ready. This reduces the memory footprint from
  7.6 MiB to 700 KiB.
* Only run at most one instance of xdg-dbus-proxy for IBus and one for
  the a11y bus. We don't need one per app, they are proxies to the same
  socket anyway.
* Remove references to netns, the dbus proxy code doesn't actually have
  anything to do with network namespaces (except that they might be
  necessary because the app we're running is running in a non-default
  network namespace).