lib, zebra: move "struct vrf" to be a lib module

lib, zebra: move "struct vrf" to be a lib module

Previously "struct vrf" is defined locally in zebra. Now it is moved
to be a lib module.

This is the first step to support multi-VRF in quagga. The
implementation is splitted into small patches for the purpose of
easy review.

* lib:
    "struct vrf" with basic members is defined in vrf.c. The member
    "void *info" is for user data.

    Some basic functions are defined in vrf.c for adding/deleting/
    looking up a VRF, scanning the VRF table and initializing the
    VRF module.

    The type "vrf_id_t" is defined specificly for VRF ID.

* zebra:
    The previous "struct vrf" is re-defined as "struct zebra_vrf";
    and previous "vrf" variables are renamed to "zvrf".

    The previous "struct vrf" related functions are removed from
    zbera_rib.c. New functions are defined to maintain the new
    "struct zebra_vrf".

    The names vrf_xxx are reserved for the functions in VRF module.
    So:
    - the previous vrf_table() are renamed to zebra_vrf_table();
    - the previous vrf_static_table() are renamed to
      zebra_vrf_static_table().

    The main logic is not changed.

    BTW: Add a statement to zebra_snmp.c telling that the SNMP is
         running only for the MIBs in the default VRF.

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/Makefile.am
	zebra/zebra_rib.c
	zebra/zebra_vty.c

Conflicts:
	lib/Makefile.am
	lib/memtypes.c
	zebra/rib.h
	zebra/zebra_rib.c
	zebra/zebra_rnh.c
	zebra/zebra_rnh.h
	zebra/zebra_vty.c