Simplified sendfile_max_chunk handling.

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).