Proxy ND (RFC4389 - or a sub-set thereof). This allows the 'emulation' of bridging...

Proxy ND (RFC4389 - or a sub-set thereof). This allows the 'emulation' of bridging. That is hosts in one sub-net reachable via differenet interfaces.

Introducate a new API command:
  ip6 nd proxy <host-address> <interface>

this indicates 2 things;
 1) that host <host-address> is reachable out of interface <interface>. VPP will thus install that route.
 2) NS requests sent to <host-address> will be responeded to (i.e. proxied).

Change-Id: I863f967fdb5097ab3b574769c70afdbfc8d5478a
Signed-off-by: Neale Ranns <nranns@cisco.com>