ovs-router: Add "ovs/route/lookup" command

This command is useful at least for testing.

Example output:
    % ovs-appctl ovs/route/lookup '10.0.0.1'
    gateway 172.17.0.254
    dev wm0
    %
ovs-router: Call route_table_register as it relies on route-table module

At least BSD implementation needs this.
route-table-bsd: Provide gateway info

For userspace tunneling.