lib, vtysh: support multiple VRFs by using linux netns

We realize VRFs with linux netns by default. The main job is
to associate a VRF with a netns. Currently this is done by
the configuration:

  [no] vrf N netns <netns-name>

This command is also available in vtysh and goes to only
zebra, because presently only zebra supports multiple VRF.

A file descriptor is added to "struct vrf". This is for the
associated netns file. Once the command "vrf N netns NAME"
is executed, the specified file is opened and the file
descriptor is stored in the VRF N. In this way the
association is formed.

In vrf_socket(), we first switch to the specified VRF by
using the stored file descriptor, and then can allocate
a socket which is working in the associated netns.

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>
diff --git a/lib/command.h b/lib/command.h
index a36a524..bb0122f 100644
--- a/lib/command.h
+++ b/lib/command.h
@@ -68,6 +68,7 @@
   AUTH_ENABLE_NODE,		/* Authentication mode for change enable. */
   ENABLE_NODE,			/* Enable node. */
   CONFIG_NODE,			/* Config node. Default mode of config file. */
+  VRF_NODE,			/* VRF node. */
   SERVICE_NODE, 		/* Service node. */
   DEBUG_NODE,			/* Debug node. */
   AAA_NODE,			/* AAA node. */