Install dbus policy in /usr instead of /etc (Closes: #1031550)
avahi-daemon installs the avahi-dbus.conf policy
in /etc/dbus-1. Since Debian 9 the standard directory for
package-installed dbus policies is /usr/share/dbus-1.
See: https://bugs.debian.org/1006631
Fixes: lintian: dbus-policy-in-etc