ifnet/API: Move struct ifnet definition to a <net/if_private.h>
Hide the ifnet structure definition, no user serviceable parts inside,
it's a netstack implementation detail. Include it temporarily in
<net/if_var.h> until all drivers are updated to use the accessors
exclusively.
Reviewed by: glebius
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38046