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>
diff --git a/lib/Makefile.am b/lib/Makefile.am
index fc02312..ac51fc6 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -13,7 +13,7 @@
 	sockunion.c prefix.c thread.c if.c memory.c buffer.c table.c hash.c \
 	filter.c routemap.c distribute.c stream.c str.c log.c plist.c \
 	zclient.c sockopt.c smux.c agentx.c snmp.c md5.c if_rmap.c keychain.c privs.c \
-	sigevent.c pqueue.c jhash.c memtypes.c workqueue.c
+	sigevent.c pqueue.c jhash.c memtypes.c workqueue.c vrf.c
 
 BUILT_SOURCES = memtypes.h route_types.h gitversion.h
 
@@ -28,7 +28,7 @@
 	str.h stream.h table.h thread.h vector.h version.h vty.h zebra.h \
 	plist.h zclient.h sockopt.h smux.h md5.h if_rmap.h keychain.h \
 	privs.h sigevent.h pqueue.h jhash.h zassert.h memtypes.h \
-	workqueue.h route_types.h libospf.h
+	workqueue.h route_types.h libospf.h vrf.h
 
 noinst_HEADERS = \
 	plist_int.h