Direct BFD sessions needs to be dispatched not only by IP addresses, but
also by interfaces, in order to avoid collisions between neighbors with
the same IPv6 link-local addresses.
Extend BFD session hash_ip key by interface index to handle that. Use 0
for multihop sessions.
Thanks to Sebastian Hahn for the original patch.
BFD session options are configured per interface in BFD protocol. This
patch allows to specify them also per-request in protocols requesting
sessions (currently limited to BGP).
Implement BFD authentication (part of RFC 5880). Supports plaintext
passwords and cryptographic MD5 / SHA-1 authentication.
Based on former commit from Pavel Tvrdik
