zebra: MAC and Neighbor (ARP/ND) handling

zebra: MAC and Neighbor (ARP/ND) handling

Implement handling of MACs and Neighbors (ARP/ND entries) in zebra:
- MAC and Neighbor database handlers
- Read MACs and Neighbors from the kernel, when needed and create
entries in zebra's MAC and Neighbor databases.
- Handle add/update/delete notifications from the kernel for MACs and
Neighbors and update zebra's database appropriately
- Inform locally learnt MACs and Neighbors to client
- Handle MACIP add/delete from client and install appriporiate entries
into the kernel
- Since Neighbor entries will be installed on an SVI, implement the
needed mappings

NOTE: kernel interface is only implemented for Linux/netlink

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>