ifnet/API: Move the IfAPI from if_var.h to if.h

Summary:
The "public" KPI for ifnet belongs in net/if.h, with net/if_var.h being
implementation details for the netstack. This is the next step in
enforcing that separation.

Reviewed by: melifaro
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38030