Bug 509446 - portable blocking gio cancellation

	* gcancellable.c (g_cancellable_make_pollfd): New method to make a
	GPollFD for a cancellable (which is slightly more complicated on
	Windows than Unix).

	* gunixinputstream.c (g_unix_input_stream_read):
	* gunixoutputstream.c (g_unix_output_stream_write): Use
	g_cancellable_make_pollfd() and g_poll() rather than using poll()
	directly.

	* tests/unix-streams.c: test of GUnixInputStream,
	GUnixOutputStream, and GCancellable.

svn path=/trunk/; revision=7553