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/zebra_rib.c b/zebra/zebra_rib.c
index 5c75b90..2997712 100644
--- a/zebra/zebra_rib.c
+++ b/zebra/zebra_rib.c
@@ -170,6 +170,31 @@
   return vrf->stable[afi][safi];
 }
 
+/*
+ * nexthop_type_to_str
+ */
+const char *
+nexthop_type_to_str (enum nexthop_types_t nh_type)
+{
+  static const char *desc[] = {
+    "none",
+    "Directly connected",
+    "Interface route",
+    "IPv4 nexthop",
+    "IPv4 nexthop with ifindex",
+    "IPv4 nexthop with ifname",
+    "IPv6 nexthop",
+    "IPv6 nexthop with ifindex",
+    "IPv6 nexthop with ifname",
+    "Null0 nexthop",
+  };
+
+  if (nh_type >= ZEBRA_NUM_OF (desc))
+    return "<Invalid nh type>";
+
+  return desc[nh_type];
+}
+
 /* Add nexthop to the end of the list.  */
 static void
 nexthop_add (struct rib *rib, struct nexthop *nexthop)