Don't provide an RTA_GATEWAY sockaddr when we write RTM_CHANGE messages
to the routing socket.

The local address on a point-to-point interface is not actually a
gateway address - despite it appearing in the second column of
netstat -r's output. Providing a gateway to an RTM_CHANGE will
currently change the route's interface so that it's using the
specified gateway - not what we want.

Patiently explained to me by: ru