bgpd: Rejiggle exported nht function names and consolidate some code

* bgp_nht.h: Tweak the API a bit to simplify and make names a bit clearer on
  function. Remove AFI argument, it's implied in both bgp_infos and peers.

  (bgp_find_nexthop) this doesn't so much find a bnc, as check the bnc
  for the given bgp_info is valid. Rename to (bgp_nexthop_check).

  (bgp_find_or_add_nexthop) This ensures a bnc exists, so call it
  (bgp_ensure_nexthop).

  (bgp_unlink_nexthop_by_peer) Remove via peer.

* bgp_nht.c: Adjust to above.
  (bgp_get_nexthop_rn) helper to get the rn.
  (bgp_find_nexthop) further helper to get the bnc for path or peer.
  (bgp_unlink_nexthop_check) helper to check whether a bnc should go.
  (bgp_ensure_nexthop) Use the helpers.

* bgp_{route,fsm}.c: s/bgp_find_or_add_nexthop/bgp_ensure_nexthop/
diff --git a/bgpd/bgp_fsm.c b/bgpd/bgp_fsm.c
index 67c50c4..9d86655 100644
--- a/bgpd/bgp_fsm.c
+++ b/bgpd/bgp_fsm.c
@@ -507,7 +507,7 @@
       /* Reset peer synctime */
       peer->synctime = 0;
     }
-
+  
   /* Stop read and write threads when exists. */
   BGP_READ_OFF (peer->t_read);
   BGP_WRITE_OFF (peer->t_write);
@@ -720,8 +720,7 @@
       ! CHECK_FLAG (peer->flags, PEER_FLAG_DISABLE_CONNECTED_CHECK))
     connected = 1;
 
-  bgp_find_or_add_nexthop(family2afi(peer->su.sa.sa_family), NULL, peer,
-			  connected);
+  bgp_ensure_nexthop (NULL, peer, connected);
   status = bgp_connect (peer);
 
   switch (status)