Major restructure to method signatures required to handle multiple head ...

...requests but little logic changes.  Renamed "head_response" attribute to "server_info" since this represents the superset of HEAD responses received from a server rather than just a single HEAD response.  Made request, response, and structure strings temporary since they can vary per HEAD.  Modified logic in dlna_src_init_uri() so it will update the overall server info with the second HEAD response.