lib, zebra, vtysh: configure an interface in non-default VRF

Introduce a new command "interface IFNAME vrf N" to configure an
interface in the non-default VRF.

Till now, only zebra uses this command. Other daemons will install
the command when they support multiple VRFs.

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/lib/if.h b/lib/if.h
index 03bd521..ad85dca 100644
--- a/lib/if.h
+++ b/lib/if.h
@@ -342,6 +342,8 @@
 extern struct cmd_element no_interface_desc_cmd;
 extern struct cmd_element interface_cmd;
 extern struct cmd_element no_interface_cmd;
+extern struct cmd_element interface_vrf_cmd;
+extern struct cmd_element no_interface_vrf_cmd;
 extern struct cmd_element interface_pseudo_cmd;
 extern struct cmd_element no_interface_pseudo_cmd;
 extern struct cmd_element show_address_cmd;