The ngx_linux_sendfile() function is now used for both normal sendfile()
and sendfile in threads.  The ngx_linux_sendfile_thread() function was
modified to use the same interface as ngx_linux_sendfile(), and is simply
called from ngx_linux_sendfile() when threads are enabled.

Special return code NGX_DONE is used to indicate that a thread task was
posted and no further actions are needed.

If number of bytes sent is less that what we were sending, we now always
retry sending.  This is needed for sendfile() in threads as the number
of bytes we are sending might have been changed since the thread task
was posted.  And this is also needed for Linux 4.3+, as sendfile() might
be interrupted at any time and provides no indication if it was interrupted
or not (ticket #1174).

This has somehow escaped from fbdaad9b0e7b.

The directive configures a timeout to be used when gracefully shutting down
worker processes.  When the timer expires, nginx will try to close all
the connections currently open to facilitate shutdown.

There is no need to cancel timers early if there are other timers blocking
shutdown anyway.  Preserving such timers allows nginx to continue some
periodic work till the shutdown is actually possible.

With the new approach, timers with ev->cancelable are simply ignored when
checking if there are any timers left during shutdown.

These messages doesn't seem to be needed in practice and only make
debugging logs harder to read.

The ngx_chain_coalesce_file() function may produce more bytes to send then
requested in the limit passed, as it aligns the last file position
to send to memory page boundary.  As a result, (limit - send) may become
negative.  This resulted in big positive number when converted to size_t
while calling ngx_output_chain_to_iovec().

Another part of the problem is in ngx_chain_coalesce_file(): it changes cl
to the next chain link even if the current buffer is only partially sent
due to limit.

Therefore, if a file buffer was not expected to be fully sent due to limit,
and was followed by a memory buffer, nginx called sendfile() with a part
of the file buffer, and the memory buffer in trailer.  If there were enough
room in the socket buffer, this resulted in a part of the file buffer being
skipped, and corresponding part of the memory buffer sent instead.

The bug was introduced in 8e903522c17a (1.7.8).  Configurations affected
are ones using limits, that is, limit_rate and/or sendfile_max_chunk, and
memory buffers after file ones (may happen when using subrequests or
with proxying with disk buffering).

Fix is to explicitly check if (send < limit) before constructing trailer
with ngx_output_chain_to_iovec().  Additionally, ngx_chain_coalesce_file()
was modified to preserve unfinished file buffers in cl.

The new parameters "manager_files", "manager_sleep"
and "manager_threshold" were added to proxy_cache_path
and friends.

Note that ngx_path_manager_pt was changed to return ngx_msec_t
instead of time_t (API change).