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_interface.h b/ospfd/ospf_interface.h
index ab0b758..f495c49 100644
--- a/ospfd/ospf_interface.h
+++ b/ospfd/ospf_interface.h
@@ -73,6 +73,9 @@
   
   DECLARE_IF_PARAM (struct list *, auth_crypt);    /* List of Auth cryptographic data. */
   DECLARE_IF_PARAM (int, auth_type);               /* OSPF authentication type */
+  
+  /* Other, non-configuration state */
+  u_int32_t network_lsa_seqnum;		/* Network LSA seqnum */
 };
 
 enum
@@ -167,6 +170,7 @@
 
   /* Configured varables. */
   struct ospf_if_params *params;
+  
   u_int32_t crypt_seqnum;		/* Cryptographic Sequence Number */ 
   u_int32_t output_cost;	        /* Acutual Interface Output Cost */