Port glean neighbor entry support to IPv6

Port glean neighbor entry support to IPv6

This patch is more or less a port of I71f3ba0c8192 to IPv6.
In practice it allows creating a route via a neighbor which is not resolved yet.
It also adds static flag to IPv6 neighbor entries.
And as Damjan suggested, it formalizes ip46_address_t by using
the IPv4 embedded IPv6 address format.

Change-Id: Ifa7328a03380ea4ff118b7ca4897b4ab23a3e57c
Signed-off-by: Pierre Pfister <ppfister@cisco.com>