serve: use chunked encoding in hgweb responses 'hg serve' used to close connections when sending a response with unknown length ... such as a bundle or archive. Now chunked encoding will be used for responses with unknown length, and the connection do thus not have to be closed to indicate the end of the response. Chunked encoding is only used if the length is unknown, if the connection wouldn't be closed for other reasons, AND if it is a HTTP 1.1 request. This will not benefit other users of hgweb ... but it can serve as an example that it can be done.