* prj-attr.adb: Attribute names Initial_Required_Switches and
Final_Required_Switches changed to Leading_Required_Switches and
Trailing_Required_Switches.
* prj-nmsc.adb (Process_Compiler): Attribute names
Initial_Required_Switches and Final_Required_Switches changed to
Leading_Required_Switches and Trailing_Required_Switches.
* prj.ads (Language_Config): Component Initial_Required_Switches and
Final_Required_Switches changed to Leading_Required_Switches and
Trailing_Required_Switches.
* snames.ads-tmpl: Remove standard names Initial_Required_Switches and
Final_Required_Switches; add standard names Leading_Required_Switches
and Trailing_Required_Switches.