MFC: r312687, r312916

MFC: r312687, r312916

Approved by:	sephe (mentor)

r312687
    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

r312916
    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

git-svn-id: svn://svn.freebsd.org/base/stable/10@314122 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f