Dear Maintainer,

Some systems have network interfaces that are only sometimes present -
for example, VPN links.  It would be useful to include these
interfaces in the fwbuilder configuration and have rules matching them
(with the intention that they never match when the interface is not
present).

However, doing this at present means that if the generated firewall
script is executed when the interface is _not_ present, the whole
script will fail and not establish the firewall properly on any of the
interfaces.