The ngx_open_dir() function changed to restore name passed to it.  This
fixes removing destination directory in dav module, as caught by dav.t.
The ngx_close_dir() function introduced to properly convert errors, as
FindClose() returns 0 on error.

Previous workaround to avoid warnings on OS X due to deprecated system
OpenSSL library (introduced in a3870ea96ccd) no longer works, as
the MAC_OS_X_VERSION_MIN_REQUIRED macro is ignored on OS X 10.9
if a compiler used supports __attribute__(availability).

This fixes --with-file-aio support on systems that lack eventfd()
syscall, notably aarch64 Linux.

The syscall(SYS_eventfd) may still be necessary on systems that
have eventfd() syscall in the kernel but lack it in glibc, e.g.
as seen in the current CentOS 5 release.

If set, its value is output in "nginx -v" and in the error log.

The "ngx_quit" may be reset by the worker thread before it's seen
by a ngx_cache_manager_thread(), resulting in an infinite loop.  Make
sure to test ngx_exiting as well.

This brings Cygwin compilation in line with other case-insensitive
systems (notably win32 and OS X) where one can force case sensitivity
using -DNGX_HAVE_CASELESS_FILESYSTEM=0.

Warnings about GetVersionEx() deprecation silenced.  Precompiled object
linked in.