In case a subnet is moved from one interface to another the policies can
remain as is but the route has to change. This currently doesn't happen
automatically and there is no option to update the policy or route so
removing and reinstalling the policies is the only option.
Fixes#2820.
After an interface disappeared we can't remove the policies correctly as
the name doesn't resolve to the previous index anymore.
And making the policies so specific might not provide that much benefit.
To handle the interfaces on the policies correctly would require some
changes to the child-cfg, kernel-interface etc. so they'd take interface
indices directly so we could target the policies correctly even if an
interface disappeared (or reappeared and got a new index).
Tobias Brunner