ospfd: Remember network LSA sequence numbers across up/downs of an interface

* ospf_interface.h: (struct ospf_if_params) add field for saved network LSA
  seqnum
* ospf_interfa.c: (ospf_new_if_params) init network_lsa_seqnum field to
  initial seqnum - doesnt matter though.
* ospf_lsa.c: (ospf_network_lsa_new) check for any saved sequence number,
  and use if it exists.  Save the result back. This should help avoid needless
  round of LSUpdate/LSRequests when a neighbour has to tell the originator
  "uhm, i have something newer than that already".
* ospf_vty.c: (show_ip_ospf_interface_sub) Show the saved network LSA seqnum
diff --git a/ospfd/ospf_vty.c b/ospfd/ospf_vty.c
index adc822a..4e52a11 100644
--- a/ospfd/ospf_vty.c
+++ b/ospfd/ospf_vty.c
@@ -2933,7 +2933,13 @@
 		       inet_ntoa (nbr->address.u.prefix4), VTY_NEWLINE);
 	    }
 	}
-
+      
+      /* Next network-LSA sequence number we'll use, if we're elected DR */
+      if (oi->params && ntohl (oi->params->network_lsa_seqnum)
+                          != OSPF_INITIAL_SEQUENCE_NUMBER)
+        vty_out (vty, "  Saved Network-LSA sequence number 0x%x%s",
+                 ntohl (oi->params->network_lsa_seqnum), VTY_NEWLINE);
+      
       vty_out (vty, "  Multicast group memberships:");
       if (OI_MEMBER_CHECK(oi, MEMBER_ALLROUTERS)
           || OI_MEMBER_CHECK(oi, MEMBER_DROUTERS))