*: unify if_is_loopback/if_is_loopback_or_vrf

*: unify if_is_loopback/if_is_loopback_or_vrf

We should always treat the VRF interface as a loopback. Currently, this
is not the case, because in some old pre-VRF code we use if_is_loopback
instead of if_is_loopback_or_vrf. To avoid any future problems, the
proposal is to rename if_is_loopback_or_vrf to if_is_loopback and use it
everywhere. if_is_loopback is renamed to if_is_loopback_exact in case
it's ever needed, but currently it's not used anywhere.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>