Clearing cache based on free space left on a file system is
expected to allow better disk utilization in some cases, notably
when disk space might be also used for something other than nginx
cache (including nginx own temporary files) and while loading
cache (when cache size might be inaccurate for a while, effectively
disabling max_size cache clearing).

Based on a patch by Adam Bambuch.

With MinGW-w64, building 64-bit nginx binary with GCC 8 and above
results in warning due to cast of GetProcAddress() result to ngx_wsapoll_pt,
which GCC thinks is incorrect.  Added intermediate cast to "void *" to
silence the warning.

FormatMessage() seems to return many errors which essentially indicate that
the language in question is not available.  At least the following were
observed in the wild and during testing: ERROR_MUI_FILE_NOT_FOUND (15100)
(ticket #1868), ERROR_RESOURCE_TYPE_NOT_FOUND (1813).  While documentation
says it should be ERROR_RESOURCE_LANG_NOT_FOUND (1815), this doesn't seem
to be the case.

As such, checking error code was removed, and as long as FormatMessage()
returns an error, we now always try the default language.

This makes it possible to avoid looping for a long time while working
with a fast enough peer when data are added to the socket buffer faster
than we are able to read and process them (ticket #1431).  This is
basically what we already do on FreeBSD with kqueue, where information
about the number of bytes in the socket buffer is returned by
the kevent() call.

With other event methods rev->available is now set to -1 when the socket
is ready for reading.  Later in ngx_recv() and ngx_recv_chain(), if
full buffer is received, real number of bytes in the socket buffer is
retrieved using ioctl(FIONREAD).  Reading more than this number of bytes
ensures that even with edge-triggered event methods the event will be
triggered again, so it is safe to stop processing of the socket and
switch to other connections.

Using ioctl(FIONREAD) only after reading a full buffer is an optimization.
With this approach we only call ioctl(FIONREAD) when there are at least
two recv()/readv() calls.

WSAPoll() is only available with Windows Vista and newer (and only
available during compilation if _WIN32_WINNT >= 0x0600).  To make
sure the code works with Windows XP, we do not redefine _WIN32_WINNT,
but instead load WSAPoll() dynamically if it is not available during
compilation.

Also, sockets are not guaranteed to be small integers on Windows.
So an index array is used instead of NGX_USE_FD_EVENT to map
events to connections.

Previous interface of ngx_open_dir() assumed that passed directory name
has a room for NGX_DIR_MASK at the end (NGX_DIR_MASK_LEN bytes).  While all
direct users of ngx_dir_open() followed this interface, this also implied
similar requirements for indirect uses - in particular, via ngx_walk_tree().

Currently none of ngx_walk_tree() uses provides appropriate space, and
fixing this does not look like a right way to go.  Instead, ngx_dir_open()
interface was changed to not require any additional space and use
appropriate allocations instead.

On some systems, it's possible that reaper of orphaned processes is
set to something other than "init" process.  On such systems, the
changing binary procedure did not work.

The fix is to check if PPID has changed, instead of assuming it's
always 1 for orphaned processes.

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.