Allow configuration of BGP sessions to non standard ports.

Hi, The current configuration options assume that one or both BGP peers will be listening on port 179. As a result it becomes impossible to configure BIRD instances to talk to each other if both are running BGP on non standard ports, or if a single instance is on a non standard port and running in passive mode. I've attached a patch which I think does the trick, allowing the user to specify a 'neighbor port <portnum>;' option, but I'm not that familiar with the codebase so I might be missing something. If the patch is not appropriate in its current form, please consider this a feature request for future versions. Thanks, Joao.