[Ada] GNAT.OS_Lib clean ups/improvements

[Arnaud Charlet <charlet at adacore dot com>]

Tested on x86-linux + manual checks on ppc-darwin and x86-windows
Committed on mainline.

Various clean ups related to GNAT.OS_Lib:

First, an issue appeared under Windows 2000 where launching
gnatmake in low priority (start /low gnatmake) launched gcc
subprocesses at normal priorities instead of inheriting gnatmake's
priority.

Then, we're adding in package GNAT.OS_Lib two new Spawn functions.
These procedures redirect the output from the spawned
program into either a file descriptor, or a file.

Finally, fix the fact that GNAT.OS_Lib.Is_Symbolic_Link did not work under
Darwin.

2004-11-18  Arnaud Charlet  <charlet@adacore.com>

	* adaint.h, adaint.c
	(__gnat_portable_spawn): Fix cast of spawnvp third parameter
	to avoid warnings.
	Add also a cast to kill another warning.
	(win32_no_block_spawn): Initialize CreateProcess's dwCreationFlags
	parameter with the priority class of the parent process instead of
	always using the NORMAL_PRIORITY_CLASS.
	(__gnat_dup): New function.
	(__gnat_dup2): New function.
	(__gnat_is_symbolic_link): Enable the effective body of this
	function when __APPLE__ is defined.

	* g-os_lib.ads, g-os_lib.adb (Spawn): Two new procedures.
	Update comments.

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