IfAPI: Add iterator to loop over all interfaces

Summary:
Sometimes it's useful to iterate over all interfaces in the current
VNET, as the linuxulator does in several places.

Unlike other iterators in the IfAPI this propagates any error received
up to the caller, instead of returning a count.

Sponsored by: Juniper Networks, Inc.
Reviewed by: glebius, melifaro
Differential Revision: https://reviews.freebsd.org/D38348