networkd: link - drop foreign config when configuring link

networkd: link - drop foreign config when configuring link

This is a change in behavior:

Before we would never remove any state, only add to it, we now drop unwanted
state from any link the moment we start managing it.

Note however, that we still will not remove any foreign state added at runtime,
to avoid any feedback loops. However, we make no guarantees about coexisting
with third-party tools that change the state of the links we manage.