src/firewall-manager tracks whether firewall is on the bus or not.
In nm-device.c at stage5 (ip-config-commit) before we actually
apply the IP configuration to the interface, we send the
IP interface name and zone to firewall and asynchronously wait
for a D-Bus reply. Then after we get the reply
(or if the firewall isn't running) we proceed with
applying the IP configuration to the interface.
