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>
diff --git a/zebra/rib.h b/zebra/rib.h
index 4f99d71..4ecfaa0 100644
--- a/zebra/rib.h
+++ b/zebra/rib.h
@@ -305,6 +305,7 @@
   rib_tables_iter_state_t state;
 } rib_tables_iter_t;
 
+extern const char *nexthop_type_to_str (enum nexthop_types_t nh_type);
 extern struct nexthop *nexthop_ifindex_add (struct rib *, unsigned int);
 extern struct nexthop *nexthop_ifname_add (struct rib *, char *);
 extern struct nexthop *nexthop_blackhole_add (struct rib *);