MFC r272889 and r287402:
- Virtualize if_epair(4). An if_xname check for both "a" and "b" interfaces
is added to return EEXIST when only "b" interface exists---this can happen
when epair<N>b is moved to a vnet jail and then "ifconfig epair<N> create"
is invoked there.
- Fix a panic which was reproducible by an infinite loop of
"ifconfig epair0 create && ifconfig epair0a destroy".
This was caused by an uninitialized function pointer in
softc->media.
git-svn-id: svn://svn.freebsd.org/base/stable/10@287594
ccf9f872-aa2e-dd11-9fc8-
001c23d0bc1f