zebra, lib/memtypes.c: the netlink sockets work per VRF

This patch lets the netlink sockets work per VRF.

* The definition of "struct nlsock" is moved into zebra/rib.h.

* The previous global variables "netlink" and "netlink_cmd" now
  become the members of "struct zebra_vrf", and are initialized
  in zebra_vrf_alloc().

* All relative functions now work for a specific VRF, by adding
  a new parameter which specifies the working VRF, except those
  functions in which the VRF ID can be obtained from the interface.

* kernel_init(), interface_list() and route_read() are now also
  working per VRF, and moved from main() to zebra_vrf_enable().

* A new function kernel_terminate() is added to release the
  netlink sockets. It is called from zebra_vrf_disable().

* Correct VRF ID, instead of the previous VRF_DEFAULT, are now
  passed to the functions of processing interfaces or route
  entries.

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: Donald Sharp <sharpd@cumulusnetworks.com>
diff --git a/zebra/rtread_sysctl.c b/zebra/rtread_sysctl.c
index b8f5bde..385e150 100644
--- a/zebra/rtread_sysctl.c
+++ b/zebra/rtread_sysctl.c
@@ -24,6 +24,7 @@
 
 #include "memory.h"
 #include "log.h"
+#include "vrf.h"
 
 #include "zebra/zserv.h"
 #include "zebra/rt.h"
@@ -31,7 +32,7 @@
 
 /* Kernel routing table read up by sysctl function. */
 void
-route_read (void)
+route_read (struct zebra_vrf *zvrf)
 {
   caddr_t buf, end, ref;
   size_t bufsiz;
@@ -47,7 +48,10 @@
     NET_RT_DUMP,
     0
   };
-		      
+
+  if (zvrf->vrf_id != VRF_DEFAULT)
+    return;
+
   /* Get buffer size. */
   if (sysctl (mib, MIBSIZ, NULL, &bufsiz, NULL, 0) < 0) 
     {