Zebra: Remove reliance on NEXTHOP_TYPE_IPV4_ONLINK

Zebra: Remove reliance on NEXTHOP_TYPE_IPV4_ONLINK

Zebra already knows if an interface is unnumbered or not.  This
is communicated to OSPF.
OSPF would only send a NEXTHOP_TYPE_IPV4_ONLINK *if* the path
was unnumbered, which it learns from Zebra.

As such, Have OSPF use the normal NEXTHOP_TYPE_IPV4_IFINDEX
type for unnumbered paths.  In Zebra, if the ifindex recieved
is unnumbered then assume that the link is NEXTHOP_FLAG_ONLINK.

Ticket: CM-8145
Reviewed-by: CCR-3771
Testing: See bug

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