zebra: maintain RTADV per VRF

This moves the global variable "rtadv" into the "struct zebra_vrf",
so that RTADV feature can work per VRF.

* rtadv.c/rtadv.h:

  Add a proper parameter to the functions so that the entity of the
  "struct zebra_vrf" and interfaces can be obtained from the specified
  VRF.

  The old rtadv_init() is splitted into:
  - rtadv_cmd_init(): it installs the RTADV commands; is called from
                      main();
  - new rtadv_init(): it creates the socket; is called from
                      zebra_vrf_enable().

  rtadv_terminate() is added to stop the threads, close the socket and
  clear the counters. It is called from zebra_vrf_disable().

  rtadv_make_socket() now calls vrf_socket() to create a socket in
  the VRF.

* interface.h and rib.h: define the macro RTADV.
* main.c: according changes, refer to rtadv.c.

Signed-off-by: Feng Lu <lu.feng@6wind.com>
Reviewed-by: Alain Ritoux <alain.ritoux@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Vincent JARDIN <vincent.jardin@6wind.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/zebra/interface.h b/zebra/interface.h
index 0f081f6..fe24604 100644
--- a/zebra/interface.h
+++ b/zebra/interface.h
@@ -38,11 +38,13 @@
 #define IF_ZEBRA_SHUTDOWN_ON     1
 
 /* Router advertisement feature. */
+#ifndef RTADV
 #if (defined(LINUX_IPV6) && (!defined(__GLIBC__) || defined(__GLIBC__) && __GLIBC__ >= 2 && __GLIBC_MINOR__ >= 1)) || defined(KAME)
   #ifdef HAVE_RTADV
     #define RTADV
   #endif
 #endif
+#endif
 
 #ifdef RTADV
 /* Router advertisement parameter.  From RFC4861, RFC6275 and RFC4191. */