don't know the netmask. (It also lets you test, at compile time,
whether you can rely on "ip broadcast" failing to compile when you pass
0xffffffff to pcap_compile().)
(255.255.255.255) be an indication that the netmask is unknown, and
return an error. Document that as the way to tell pcap_compile() that
the netmask is unknown. Have filtertest default to that as the netmask,
and add a -m flag to let you specify the netmask.
Guy Harris