network: drop unnecessary routing policy rules

networkd already drop foreign address, routes, and nexthops on startup,
except those created by kernel. However, previously, routing policy
rules were not. The logic of serialization/deserialization of rules only
works for rules created by previous invocation of networkd, and does not
work for one created by other tools like `ip rule`.

This makes networkd drop foreign routing policy rules except created by
kernel on startup. Also, remove rules created by networkd when the
corresponding links are dropped or networkd is stopping.