networkd: netdev - inform when we take over an existing netdev

networkd: netdev - inform when we take over an existing netdev

The crucial point here is that we will not change the settings of a netdev created by someone else
we simply use it as is and trust it was set up as intended.

This is confusing in the case of the pre-created netdev's (bond0 etc.), the solution should probably
be to simply make the kernel stop creating these devices as they are pretty useless.