ospfd: export ospf_if_table_lookup() and use it in ospf_network_run_interface()

Makes it possible to run OSPF on multiple PtP interfaces
with the same remote address.

* ospfd/ospf_interface.c: Export ospf_if_table_lookup().
* ospfd/ospf_interface.h: ditto.
* ospfd/ospfd.c: (ospf_network_run_interface) Use ospf_if_table_lookup() to
  determine whether OSPF is already configured for a subnet and interface.

diff --git a/ospfd/ospf_interface.h b/ospfd/ospf_interface.h
index 79b178d..0fc4ccb 100644
--- a/ospfd/ospf_interface.h
+++ b/ospfd/ospf_interface.h
@@ -248,6 +248,8 @@
 							    struct in_addr);
 extern struct ospf_interface *ospf_if_lookup_by_prefix (struct ospf *,
 							struct prefix_ipv4 *);
+extern struct ospf_interface *ospf_if_table_lookup (struct interface *,
+						    struct prefix *);
 extern struct ospf_interface *ospf_if_addr_local (struct in_addr);
 extern struct ospf_interface *ospf_if_lookup_recv_if (struct ospf *,
 						      struct in_addr);