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

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.