(__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.

From-SVN: r90899