build-sys: stop placing D-Bus policy below /etc

build-sys: stop placing D-Bus policy below /etc

Let's start placing our D-Bus policy files in /usr rather than /etc. D-Bus
supports this since 1.9.18, and moving our files over means we continue to work
even if /etc is flushed out entirely (for example if systemd-nspawn's
--volatile= switch is used).

Since 1.9.18 was released summer 2015 it should be fine to require a newer
version like this for our builds.

https://lists.freedesktop.org/archives/dbus/2015-July/016746.html