Also apply x-forwarded-for on source connections.

Hi Karl,

I am load-balancing incoming source connections through HAProxy. This patch add's support for x-forwarded-for for source clients. This is done by factoring out the x-forwarded-for handling into a new function _check_for_x_forwarded_for. This function is then used in _handle_source_request in addition to the existing use in _handle_get_request.

Is this something that you would consider merging into icecast-kh?

Kind Regards,
Klaas Jan