zebra: link layer config and notification, implementation in zebra

zebra: link layer config and notification, implementation in zebra

zebra implements zebra api for configuring link layer information. that
can be an arp entry (for ipv4) or ipv6 neighbor discovery entry. This
can also be an ipv4/ipv6 entry associated to an underlay ipv4 address,
as it is used in gre point to multipoint interfaces.
this api will also be used as monitoring. an hash list is instantiated
into zebra (this is the vrf bitmap). each client interested in those entries
in a specific vrf, will listen for following messages: entries added, removed,
or who-has messages.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>