This commit fixes two different bugs:
(1) First in some situations Nmap will only see routes that are attached
to the device that handles the default route.
(2) On boxes without a default route, Nmap will not see any route.
These two bugs are caused by sysroutes_dnet_find_interfaces() logic
which will use only the geteway to match interface addresses.
To fix this, first check the current route and see if the gateway was set
otherwise use the destination address to match the address of an
interface.