zebra: show interfaces in a specified VRF or all VRFs

zebra: show interfaces in a specified VRF or all VRFs

The following commands only show interfaces in the default VRF:
    show interface
    show interface IFNAME
    show interface description

New options are introduced to show interfaces in a specified VRF:
    show interface vrf N
    show interface IFNAME vrf N
    show interface description vrf N

or all VRFs:
    show interface vrf all
    show interface IFNAME vrf all
    show interface description vrf all

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>