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 XFS, using "allocsize=64m" mount option results in large preallocation
being reported in the st_blocks as returned by fstat() till the file is
closed.  This in turn results in incorrect cache size calculations and
wrong clearing based on max_size.

To avoid too aggressive cache clearing on such volumes, st_blocks values
which result in sizes larger than st_size and eight blocks (an arbitrary
limit) are no longer trusted, and we use st_size instead.

The ngx_de_fs_size() counterpart is intentionally not modified, as
it is used on closed files and hence not affected by this problem.

NFS on Linux is known to report wsize as a block size (in both f_bsize
and f_frsize, both in statfs() and statvfs()).  On the other hand,
typical file system block sizes on Linux (ext2/ext3/ext4, XFS) are limited
to pagesize.  (With FAT, block sizes can be at least up to 512k in
extreme cases, but this doesn't really matter, see below.)
To avoid too aggressive cache clearing on NFS volumes on Linux, block
sizes larger than pagesize are now ignored.

Note that it is safe to ignore large block sizes.  Since 3899:e7cd13b7f759
(1.0.1) cache size is calculated based on fstat() st_blocks, and rounding
to file system block size is preserved mostly for Windows.

Note well that on other OSes valid block sizes seen are at least up
to 65536.  In particular, UFS on FreeBSD is known to work well with block
and fragment sizes set to 65536.

Listening UNIX sockets were not removed on graceful shutdown, preventing
the next runs.  The fix is to replace the custom socket closing code in
ngx_master_process_cycle() by the ngx_close_listening_sockets() call.

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.

AIO support in nginx was originally developed against FreeBSD versions 4-6,
where the sival_ptr field was named as sigval_ptr (seemingly by mistake[1]),
which made nginx use the only name available then.  The standard-complaint
name was restored in 2005 (first appeared in FreeBSD 7.0, 2008), retaining
compatibility with previous versions[2][3].  In DragonFly, similar changes
were committed in 2009[4], with backward compatibility recently removed[5].

The change switches to the standard name, retaining compatibility with old
FreeBSD versions.

[1] https://svnweb.freebsd.org/changeset/base/48621
[2] https://svnweb.freebsd.org/changeset/base/152029
[3] https://svnweb.freebsd.org/changeset/base/174003
[4] https://gitweb.dragonflybsd.org/dragonfly.git/commit/3693401
[5] https://gitweb.dragonflybsd.org/dragonfly.git/commit/7875042

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.