When an interface doesn't have an address set, getInterfaceByName can
fail because it checks the address family. The fatal error message would
be something like
route_dst_netlink: can't find interface "tap0"
If we can't find an interface with a specific address family, fall back
with an AF_UNSPEC search.
http://seclists.org/nmap-dev/2012/q4/12
