ifnet: introduce event handlers for ifup/ifdown events

[decui_microsoft.com]

Summary

Hyper-V's NIC SR-IOV implementation needs a Hyper-V synthetic NIC and
a VF NIC to work together, mainly to support seamless live migration.

When the VF device becomes UP (or DOWN), the synthetic NIC driver needs
to switch the data path from the synthetic NIC to the VF (or the opposite).

So the synthetic NIC driver needs to know when a VF device is becoming
UP or DOWN and hence the patch is made.

[decui_microsoft.com]

Since sephe's patch (https://reviews.freebsd.org/D8905) is accepted, let me update this patch accordingly.

[sepherosa_gmail.com]

If no objection comes, we plan to commit this earlier next week.