Basic support for multiple WANs (netifd & default gateway)

[Stefan Kalscheuer]

Hi Rafał,

I'm using a similar setup for quite some time with 2 WAN interfaces on 
OpenWrt 22.03 SNAPSHOT:
* "wan" - statically routed, native dual stack fiber uplink
* "wwan" - uqmi 4G upstream with dynamically assigned dual stack (DHCP)

Configured both interfaces with "use default gateway" and gateway metric 
("metric") of 0 and 2000 respectively.

Results in default routing like this::
default via 172.16.100.1 dev eth2 proto static
default via 10.142.198.233 dev wwan0 proto static src 10.142.198.234 
metric 2000
default via 2001:****:... dev eth2 proto static metric 1024 pref medium
default via 2a02:****:... dev wwan0 proto static metric 2000 pref medium

Works for me without additional changes. Traffic is routed over "wan" as 
long as it's up and automatically switches to "wwan" when I disable the 
interface or pull the cable.

I decided against mwan3 for now, because I don't need advanced features 
either and I do have some scenarios, where I must route certain traffic 
over a specific uplink, so I got a hand full of static routes and 
scripts anyway.

There are some downsides, if the interface is not actually "down", but 
simply fails forwarding traffic or just one protocol (v4/v6) goes down.
Also both interfaces have to stay up permanently, unless you add some 
trigger scripts.

Cheers,
Stefan