gnetworkmonitornetlink: handle default route via device

gnetworkmonitornetlink: handle default route via device

If the default route is via a device rather than a particular IP
address, then neither RTA_DST nor RTA_GATEWAY will be present in the
RTM_NEWROUTE message, and so GNetworkMonitorNetlink would ignore it,
and then think there was no default route. (This could happen with
certain kinds of VPNs, if they were set to route all traffic through
the VPN.)

Fix this by recognizing routes that specify RTA_OIF ("output
interface") instead of RTA_GATEWAY.

https://bugzilla.gnome.org/show_bug.cgi?id=701609
(cherry picked from commit c08ef6c165c6935f257d3fb98c049be50e3816da)