zebra: Start breakup of zns into zrouter and zns

zebra: Start breakup of zns into zrouter and zns

The `struct zebra_ns` data structure is being used
for both router information as well as support for
the vrf backend( as appropriate ).  This is a confusing
state.  Start the movement of `struct zebra_ns` into
2 things `struct zebra_router` and `struct zebra_ns`.

In this new regime `struct zebra_router` is purely
for handling data about the router.  It has no knowledge
of the underlying representation of the Data Plane.

`struct zebra_ns` becomes a linux specific bit of code
that allows us to handle the vrf backend and is allowed
to have knowledge about underlying data plane constructs.

When someone implements a *bsd backend the zebra_vrf data
structure will need to be abstracted to take advantage of this
instead of relying on zebra_ns.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>