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

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>
Conflicts:
	zebra/interface.c