Nping calls route_dst at least twice: once with a NULL device, and again
with the device learned from the first time. This interfered with the
code that automatically chooses a loopback interface for dests that are
the same as an interface address. For example, if you are 192.168.0.1,
and you are scanning 192.168.0.1, route_dst will tell you to use
interface "lo0" even though the interface of 192.168.0.1 may be "eth0".
route_dst was returning failure because the device name check was being
done before "eth0" got changed to "lo0".
This problem didn't show up on Linux because Linux uses
route_dst_netlink, which delegates the work to the kernel. But I could
reproduce on Linux by forcing the use of route_dst_generic.
66bd9d1993