zebra: Allow recursively resolved blackhole routes to be installed

zebra: Allow recursively resolved blackhole routes to be installed

So the current code for a blackhole route assumed that you
would never want a recursively resolved blackhole to work.
Suppose you have this setup:

1) ip route 192.0.2.1/32 Null0
2) BGP installed with a route-map that rewrites the
   nexthop to 192.0.2.1.

Zebra will end up with a recursive nexthop that resolves
to the blackhole.

The original rib install function assumed that we would never
want the ability to recursively resolve a blackhole route.
Instead just handle the blackhole as part of the nexthop_num = 1
case.

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