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

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