Reload policy rules for completed connections

Reload policy rules for completed connections

The message bus which can monitor its conf dirs for changes and reload
confs immediately if dir monitor enabled, for example, inotify in Linux,
kqueue in *BSD.

However, it doesn't apply policy rules change for completed connections,
so to apply policy rules change, the client connection has to disconnect
first and then re-connect to message bus.

For imcomplete connections, it always has the latest review of policy
rules.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=39463
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>