bgpd: Extend RFC 5549 metaphor a bit more

bgpd: Extend RFC 5549 metaphor a bit more

Currently we only support RFC 5549 in bgp via
using the `neighbor swp1 interface remote-as ...`
command.  This causes the extended capability
data to be traded as part of the open message.
Additionally at that point in time we notify
zebra to turn on the RA code for that interface
so that the zebra trick of turning the v6 nexthop
into a 169.254.0.1 nexthop and adding a neighbor
entry works.

This code change does 2 things:

1) Modify bgp to pass the extended capability
if we are attempting to establish a v4/unicast
session over a v6 peer.  In the past we limited
this to just the LL based peer.

2) Modify the nexthop tracking code to notice
when it receives nexthop data about the global v6
peer to turn on RA code on those interfaces we will
be using.  This will allow the v4 route with a v6
nexthop received in zebra to auto translate this
correctly.

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