bgpd: Allow bgp to understand the different nexthop types

bgpd: Allow bgp to understand the different nexthop types

When BGP is being redistributed prefixes, allow it to
understand the nexthop type.

This fixes the issue where a blackhole route was being interpreted
to having a nexthop of 1.0.0.0( ruh-roh!!! ).  This broke
downstream neighbors as that they would receive a 1.0.0.0 nexthop,
which is bad, very very bad.

This commit sets us up for the future where we can match
a route-map against a nexthop type.  In that bgp is
now at least nominally paying attention to the type.

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