BGP: Add support for peerings with link-local v6 addresses

[Matthias Schiffer]

Hi,
I already posted a first patch to add support for link-local addresses
for BGP peerings one year ago, and now I've finally found time to
update the patch for the current bird version and fix the problems
mentioned in reply to the first patch.

This patch series allows using NEF_STICKY neighbors for link-local
addresses by saving the interface name in the neighbor structure,
thus working even when the interface is removed and re-added causing
the interface index to change.

The BPG patch itself adds an interface attribute to the BGP protocol
definition allowing to specify an interface to use for the BGP
session.

An additional patch that is needed to make this work under FreeBSD
is also included; the current behaviour doesn't handle numbered v6
addresses on peer-to-peer interfaces correctly, which link-local
addresses are generally. I didn't get the chance to test this under
other BSDs though.

The patches have been tested under Linux and FreeBSD, with Bird
and Quagga peers, and a few different configurations.

Best regards,
Matthias Schiffer