MFC r272889 and r287402:

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