Win32: added WSAPoll() support.

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.

auto/os/win32

@@ -11,6 +11,7 @@ CORE_SRCS="$WIN32_SRCS $IOCP_SRCS"
 OS_CONFIG="$WIN32_CONFIG"
 NGX_ICONS="$NGX_WIN32_ICONS"
 SELECT_SRCS=$WIN32_SELECT_SRCS
+POLL_SRCS=$WIN32_POLL_SRCS
 
 ngx_pic_opt=
 ngx_binext=".exe"

auto/sources

@@ -106,6 +106,7 @@ WIN32_SELECT_SRCS=src/event/modules/ngx_win32_select_module.c
 
 POLL_MODULE=ngx_poll_module
 POLL_SRCS=src/event/modules/ngx_poll_module.c
+WIN32_POLL_SRCS=src/event/modules/ngx_win32_poll_module.c
 
 KQUEUE_MODULE=ngx_kqueue_module
 KQUEUE_SRCS=src/event/modules/ngx_kqueue_module.c