g-socket.ads: Document the fact that Close_Selector has no effect on a closed selector.

2009-09-17  Bob Duff  <duff@adacore.com>

	* g-socket.ads: Document the fact that Close_Selector has no effect on
	a closed selector.
	* g-socket.adb: Raise an exception when a Selector that should be open
	is closed.
	(Check_Selector): Declare RSig as a constant rather than a renames,
	less confusing.

From-SVN: r151798