ipfilter: Introduce the new FORMAT_IF macro

Interface names stored in the ipstate_t and ipnat_t structures can be
NULL. This occurs when an application, such as named, is running on the
firewall machine itself. For example an application, i.e. named, running
on the firewall itself will cause a state table display and NAT mapping
display to show a null ingress interface and its egress interface. This
is perfectly valid but confusing to human eyes. Rather than print
nothing, print "(null)".

MFC after: 1 week