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_lsdb.c b/ospf6d/ospf6_lsdb.c
index 280bdf9..7455d83 100644
--- a/ospf6d/ospf6_lsdb.c
+++ b/ospf6d/ospf6_lsdb.c
@@ -572,7 +572,7 @@
   /* if current database copy not found, return InitialSequenceNumber */
   lsa = ospf6_lsdb_lookup (type, id, adv_router, lsdb);
   if (lsa == NULL)
-    seqnum = INITIAL_SEQUENCE_NUMBER;
+    seqnum = OSPF_INITIAL_SEQUENCE_NUMBER;
   else
     seqnum = (signed long) ntohl (lsa->header->seqnum) + 1;