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/vtysh/Makefile.am b/vtysh/Makefile.am
index d1ff69b..850b505 100644
--- a/vtysh/Makefile.am
+++ b/vtysh/Makefile.am
@@ -28,6 +28,7 @@
 		  $(top_srcdir)/lib/keychain.c $(top_srcdir)/lib/routemap.c \
 		  $(top_srcdir)/lib/filter.c $(top_srcdir)/lib/plist.c \
 		  $(top_srcdir)/lib/distribute.c $(top_srcdir)/lib/if_rmap.c \
+		  $(top_srcdir)/lib/vrf.c \
 		  $(top_srcdir)/lib/vty.c $(top_srcdir)/zebra/debug.c \
 		  $(top_srcdir)/zebra/interface.c \
 		  $(top_srcdir)/zebra/irdp_interface.c \