lib: updates to zapi_route

lib: updates to zapi_route

This patch introduces the following changes to the zapi_route structure
and associated code:
* Use a fixed-size array to store the nexthops instead of a pointer. This
  makes the zapi_route() function much easier to use when we have multiple
  nexthops to send. It's also much more efficient to put everything on
  the stack rather than allocating an array in the heap every time we
  need to send a route to zebra;

* Use the new 'zapi_nexthop' structure. This will allow the client daemons
  to send labeled routes without having to allocate memory for the labels
  (the 'nexthop' structure was designed to be memory efficient and doesn't
  have room for MPLS labels, only a pointer). Also, 'zapi_nexthop' is more
  compact and more clean from an API perspective;

* Embed the route prefix inside the zapi_route structure. Since the
  route's prefix is sent along with its nexthops and attributes, it makes
  sense to pack everything inside the same structure.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>