ospfd: Fix bug in 94266fa822ba, nbr_self rebuild didn't add valid nbr_self

* 94266fa822ba "ospfd: Self nbrs needs to be rebuilt when router ID changes."
  deleted the nbr_self, and added it back, but ospf_nbr_add_self doesn't
  actually create the nbr_self - it assumes it's already there. Leading
  to use after free and crashes after a router-id change.
* ospfd/ospf_neighbor.{c,h}: (ospf_nbr_self_reset) Little helper to reset the
  nbr_self correctly.
* ospf_interface.c: (ospf_if_cleanup) moved code to ospf_nbr_self_reset
* ospfd.c: (ospf_router_id_update) Use ospf_nbr_self_reset instead of doing
  the reset badly, fixing 94266fa822ba.
diff --git a/ospfd/ospfd.c b/ospfd/ospfd.c
index 019a22b..1a549c3 100644
--- a/ospfd/ospfd.c
+++ b/ospfd/ospfd.c
@@ -118,8 +118,7 @@
 	   * oi->nbr_self->router_id = router_id for
 	   * !(virtual | ptop) links
 	   */
-	  ospf_nbr_delete(oi->nbr_self);
-	  ospf_nbr_add_self(oi);
+	  ospf_nbr_self_reset (oi);
 	}
 
       /* If AS-external-LSA is queued, then flush those LSAs. */