[Ada] Better error message from GNATprove on illegal switch

[Ada] Better error message from GNATprove on illegal switch

When a compilation switch is wrongly passed to GNATprove without the
leading hyphen, this patch issues a clear error message instead of the
obscure 'usage' message previously displayed.

There is no impact on compilation.

2018-12-11  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* gnat1drv.adb (Gnat1drv): Issue specific error message in
	GNATprove mode when multiple file names on the command line.
	* osint.adb, osint.ads (Dump_Command_Line_Source_File_Names):
	New procedure to print file names on the command line.

From-SVN: r266989