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

<net/if.h> should be a fully user-facing header, so these APIs don't
belong there. Revert and will find another approach.

This reverts commit fe33e0ab83d1fbc3c5cd4a2591ba0036e47b1fec.

Fixes: fe33e0ab83d1
Sponsored by: Juniper Networks, Inc.