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.