[Ada] Improve end of command line arguments detection

[Pierre-Marie de Rodat]

Add routine Get_Argument with End_Of_Arguments Boolean out parameter
into GNAT.Command_Line API to distinguish between the empty argument and
the end of arguments.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-12-12  Dmitriy Anisimkov  <anisimko@adacore.com>

gcc/ada/

	* libgnat/g-comlin.ads (Get_Argument): New routine similar to
	original Get_Argument but with one more out parameter
	End_Of_Arguments.
	(Get_Arguments): Comment improved.
	* libgnat/g-comlin.adb (Get_Argument): Implementation taken from
	original Get_Argument and improved.
	(Get_Argument): Calls new routine Get_Argument with additional
	parameter.
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