Make gateway objects also detect routes with missing RTF_GATEWAY flag

Make gateway objects also detect routes with missing RTF_GATEWAY flag

This was a bug, not a feature. From the FreeBSD manpage rtentry(9):

| Vt struct sockaddr *rt_gateway;
|
| The "target" of the route, which can either represent a destination in
| its own right (some protocols will put a link-layer address here), or
| some intermediate stop on the way to that destination (if the
| RTF_GATEWAY flag is set).

So for a default route with "dest" and "mask" both zero, either
RTF_GATEWAY is present, or "gate" is zero.