Convert ng_iface(4) to new ifnet API.

Since this interface type is somewhat special, it still includes
if_var.h. All what ng_iface needs to know is if_transmit() macro
and access to if_index, which normal drivers shouldn't need. It
is lesser evil to make ng_iface(4) aware of ifnet structure,
rather than provide access to these fields to all drivers. Might
be rethought however.

Committed via: ng0
Sponsored by: Nginx, Inc.