lib, zebra: add "vrf_id" into the "struct interface"

lib, zebra: add "vrf_id" into the "struct interface"

Later, an interface will belong to a specific VRF. Now we add a
property "vrf_id" to the "struct interface", and keep it as the
default value 0.

This property is shown when displaying interfaces information.
It is also added in some logs.

This is just the preparation to move the interace list into the
"struct vrf". The main logic is not changed.

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:
	lib/if.c
	zebra/interface.c