[Ada] gnatcmd improvements

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk

Invoking gnat check, gnat metric and gnat pretty with a project file
(-P), with -U and with a single main will result in the invocation of the
underlying tool with the source files of the closure of the main. All the
ALI files of the closure need to exist for this to succeed.

Switch -U (VMS qualifier /ALL_PROJECTS) is now recognized by GNAT LIST.
When a project file is used, gnatls is called for all the sources of all
the projects in the project tree.

2007-06-06  Vincent Celier  <celier@adacore.com>

	* gnatcmd.adb (GNATCmd): Accept switch -aP for commands that accept
	switch -P
	(ASIS_Main): New global variable
	(Get_Closure): New procedure
	(GNATCmd): Set ASIS_Main when -P and -U with a main is used for gnat
	check, metric or pretty. Call Get_Closure in this case.
	(Check_Files): For GNAT LIST, check all sources of all projects when
	All_Projects is True.
	(GNATCmd): Accept -U for GNAT LIST

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