[Ada] g-expect fixes

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk

Raise Invalid_Process if the process id passed to Send_Signal or Close is
invalid. Otherwise, if we have a Descriptor that was not successfully
initialized (by Non_Blocking_Spawn, for example), and we try to Close it, bad
things happen -- on Windows, for example, the current process will be sent a
signal.

gnat.dg/expect1.adb should run silently

2007-06-06  Bob Duff  <duff@adacore.com>

	* g-expect-vms.adb: 
	(Send_Signal, Close): Raise Invalid_Process if the process id is invalid.
	* g-expect.ads, g-expect.adb (Send): Avoid useless copy of the string.
	(Send_Signal, Close): Raise Invalid_Process if the process id is
	invalid.
	(Pattern_Matcher_Access): Is now a general access type to be able to
	use aliased string.

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