Ada: clean ups in gnatclean tool

[Arnaud Charlet <charlet at ACT-Europe dot FR>]

Tested on x86-linux

gnatclean no longer outputs warnings for files it cannot delete
when it is in quiet output (-q) and not in verbose mode (-v).
This is consistent with the other GNAT tools.
Gnatclean now attempts to delete files only if they are writable.
Similarly to rm, a new switch -f is added so that gnatclean
attempts to delete files even when they are not writable.

2004-10-26  Vincent Celier  <celier@gnat.com>

	* clean.adb (Delete): Do not output warnings when in quiet output and
	not in verbose mode.
	(Force_Deletions): New Boolean flag, defaulted to False
	(Delete): Only delete a file if it is writable, and when
	Force_Deletions is True.
	(Parse_Cmd_Line): New switch -f: set Force_Deletions to True
	(Usage): Line for new switch -f
	(Clean_Directory): Use GNAT.OS_Lib.Set_Writable instead of rolling our
	own.

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