lib: Allow interface lookup by VRF_UNKNOWN

lib: Allow interface lookup by VRF_UNKNOWN

Modify if_lookup_by_index to accept a VRF_UNKNOWN
as a vrf_id.  This will cause it to look in all
vrf's for the interface pointer.

Subsequently all if_XXXX functions that call this function
will also get this behavior.

VRF_UNKNOWN *should* not be used for interface creation
as that this will break some core assumptions.

This work is part of allowing vrf route leaking.  Currently
it is possible to create a route in the linux kernel that has
a nexthop across vrf boundaries.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>