Simplified sendfile_max_chunk handling. Previously, it was checked that sendfile_max_chunk was enabled and almost whole sendfile_max_chunk was sent (see e67ef50c3176), to avoid delaying connections where sendfile_max_chunk wasn't reached (for example, when sending responses smaller than sendfile_max_chunk). Now we instead check if there are unsent data, and the connection is still ready for writing. Additionally we also check c->write->delayed to ignore connections already delayed by limit_rate. This approach is believed to be more robust, and correctly handles not only sendfile_max_chunk, but also internal limits of c->send_chain(), such as sendfile() maximum supported length (ticket #1870).