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/kernel_null.c b/zebra/kernel_null.c
index 4cd43db..58d2c3a 100644
--- a/zebra/kernel_null.c
+++ b/zebra/kernel_null.c
@@ -28,6 +28,7 @@
 #include "zebra/rt.h"
 #include "zebra/redistribute.h"
 #include "zebra/connected.h"
+#include "zebra/rib.h"
 
 int kernel_add_ipv4 (struct prefix *a, struct rib *b) { return 0; }
 #ifdef HAVE_SYS_WEAK_ALIAS_PRAGMA
@@ -64,9 +65,10 @@
   return 0;
 }
 
-void kernel_init (void) { return; }
+void kernel_init (struct zebra_vrf *zvrf) { return; }
+void kernel_terminate (struct zebra_vrf *zvrf) { return; }
 #ifdef HAVE_SYS_WEAK_ALIAS_PRAGMA
 #pragma weak route_read = kernel_init
 #else
-void route_read (void) { return; }
+void route_read (struct zebra_vrf *zvrf) { return; }
 #endif