zebra: extern/extract some functions from rt_netlink.c

zebra: extern/extract some functions from rt_netlink.c

* zebra/{rib.h,zebra_rib.c}

    Add nexthop_type_to_str(), which returns a human-readable string
    corresponding to a nexthop type.

  * zebra/rt_netlink.[hc]

    - Add new header file that exposes some existing and new
      netlink-related functions from rt_netlink.c to the rest of
      zebra.

        addattr32
        addattr_l
        rta_addattr_l
        nl_msg_type_to_str (new)
        nl_rtproto_to_str (new)

    - Use nexthop_type_to_str() instead of the static array
      'nexthop_types_desc'.

Signed-off-by: Avneesh Sachdev <avneesh@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>