bgpd: rename SAFI 3 according to RFC4760

- SAFI value 3 is reserved.  It was assigned by RFC 2858 for a use
  that was never fully implemented, so it is deprecated by this
  document.

* zebra.h: rename macro
* bgp_fsm.c: (bgp_graceful_restart_timer_expire,
  bgp_graceful_stale_timer_expire, bgp_stop, bgp_establish): update
* bgpd.c: (peer_nsf_stop): update
* bgp_open.c: (bgp_capability_vty_out): SAFI 3 isn't a recognized case
  any more
diff --git a/lib/zebra.h b/lib/zebra.h
index 2dc8451..6d19910 100644
--- a/lib/zebra.h
+++ b/lib/zebra.h
@@ -504,7 +504,7 @@
 /* Subsequent Address Family Identifier. */
 #define SAFI_UNICAST              1
 #define SAFI_MULTICAST            2
-#define SAFI_UNICAST_MULTICAST    3
+#define SAFI_RESERVED_3           3
 #define SAFI_MPLS_VPN             4
 #define SAFI_MAX                  5