Store a unique ID, an interface index, rather than a pointer to the

Store a unique ID, an interface index, rather than a pointer to the
receiving interface in the packet header of every mbuf.

The interface pointer should now be retrieved when necessary with
if_get().  If a NULL pointer is returned by if_get(), the interface
has probably been destroy/removed and the mbuf should be freed.

Such mechanism will simplify garbage collection of mbufs and limit
problems with dangling ifp pointers.

Tested by jmatthew@ and krw@, discussed with many.

ok mikeb@, bluhm@, dlg@