findif.sh: ignore unreachable, blackhole, and prohibit routes

[oalbrigt]

Fixes #1963.

[bootc]

If you're doing it this way I think you'll also want to ignore prohibit and blackhole routes.

I'm not entirely clear why this code does a ip route list match rather than simply a ip route get to get the route that the kernel would use. Would that not be better? Yes it works differently and it's not necessarily obvious if a route uses the default route in this case, but does it matter for this usage?

[oalbrigt]

We can ignore those as well. I dont know the reason why it was used in the first place, but it's what has been used forever.