Provide fake link status information in an attempt to let ng_eiface(4)
virtual ifnets more realistically mimic physical ethernet interfaces.
The main motivation behind this change is to allow for ng_eiface(4)
interfaces to participate in STP if_bridge(4) configurations.

When announcing link status changes, switch to the vnet to which the
ifnet belongs, since it is possible for ng_eiface ifnets to be assigned
to a vnet different from the one in which its netgraph node resides.

MFC after: 3 days