ospf6d/ospfd: refactor some common defines

Rearranging common defs and structures for use betweeen OSPFv2 and
OSPFv3.  Created a new file called libospf.h under lib directory to
hold defines that are common between OSPFv2 and OSPFv3 code bases.

[DL: split of defines refactor from timer refactor]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/ospf6d/ospf6_message.c b/ospf6d/ospf6_message.c
index 5edb70c..b35aa1a 100644
--- a/ospf6d/ospf6_message.c
+++ b/ospf6d/ospf6_message.c
@@ -1282,7 +1282,7 @@
   {
     if (IS_OSPF6_DEBUG_MESSAGE (oh->type, RECV))
     {
-      if (oh->area_id == BACKBONE_AREA_ID)
+      if (oh->area_id == OSPF_AREA_BACKBONE)
         zlog_debug ("%s: Message may be via Virtual Link: not supported", __func__);
       else
         zlog_debug