ifnet: introduce event handlers for ifup/ifdown events

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.

Reviewed by:	sephe
Approved by:	sephe (mentor)
Sponsored by:	Microsoft
Differential Revision:	https://reviews.freebsd.org/D8963

ifnet: move the new ifnet_event EVENTHANDLER_DECLARE to net/if_var.h

Thank glebius for pointing this out:
"The network stuff shall not be added to sys/eventhandler.h"

Reviewed by:	David_A_Bright_DELL.com, sephe, glebius
Approved by:	sephe (mentor)
Sponsored by:	Microsoft
Differential Revision:	https://reviews.freebsd.org/D9345