[Ada] fix VMS glitches

[Arnaud Charlet <charlet at adacore dot com>]

Manually tested on VMS
Tested on i686-linux, committed on trunk.

On VMS, the directories of the source and object paths are
displayed in VMS style (GNU:[DIR1.DIR2]).
This fix ensures that the directories in the project path
are also displayed in VMS style.
The test for this is to invoke "gnatls -v" on VMS: there should
be no directory displayed in Unix style.

Also on VMS, in absolute paths, the first directory my be a multi-valued
logical name. Expanding such a path will give only one value to
the logical name, which may not be the correct one.

2006-10-31  Vincent Celier  <celier@adacore.com>
        
	* gnatls.adb: Take into account GPR_PROJECT_PATH, when it is defined,
	instead of ADA_PROJECT_PATH, for the project path.
	(Gnatls): When displaying the project path directories, use host dir
	specs.

	* prj-ext.adb (Prj.Ext elaboration): On VMS, only expand relative path
	names in the project path, as absolute paths may correspond to
	multi-valued VMS logical names.

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