When an interface is not specified for the network module, we parse the
routing table to look for default routes.
We have defined a default route to:
- have a gateway specified, and
- have no destination specified, or have an all-zero destination.
Previous versions of Waybar had the second condition inverted,
causing it to incorrectly pick interfaces are used to route a
subnet/single host.
For example, with the following routing table, we should pick `eth0` to
show information about, not `wg0`.
```
ip -4 route
default via 192.168.252.1 dev eth0 proto dhcp src 192.168.252.200 metric 100
192.168.252.0/24 dev eth0 proto kernel scope link src 192.168.252.200 metric 100
192.168.2.254 via 192.168.1.1 dev wg0 proto static metric 50
192.168.1.0/24 dev wg0 proto static scope link metric 50
192.168.1.0/24 dev wg0 proto kernel scope link src 192.168.1.254 metric 50
```
