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.c b/ospfd/ospf_interface.c
index b333658..a812dd9 100644
--- a/ospfd/ospf_interface.c
+++ b/ospfd/ospf_interface.c
@@ -531,6 +531,8 @@
 
   oip->auth_crypt = list_new ();
   
+  oip->network_lsa_seqnum = htonl(OSPF_INITIAL_SEQUENCE_NUMBER);
+
   return oip;
 }
 
@@ -569,7 +571,8 @@
       !OSPF_IF_PARAM_CONFIGURED (oip, type) &&
       !OSPF_IF_PARAM_CONFIGURED (oip, auth_simple) &&
       !OSPF_IF_PARAM_CONFIGURED (oip, auth_type) &&
-      listcount (oip->auth_crypt) == 0)
+      listcount (oip->auth_crypt) == 0 &&
+      ntohl (oip->network_lsa_seqnum) != OSPF_INITIAL_SEQUENCE_NUMBER)
     {
       ospf_del_if_params (oip);
       rn->info = NULL;