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.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272889 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f