Shell: Add support for special parameter that expands to return-code of last program executed

Author: https://github.com/MinusGix
Commit: 91a609d945
Pull-request: https://github.com/SerenityOS/serenity/pull/537
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/bugaevc

Shell/GlobalState.h

@@ -14,6 +14,7 @@ struct GlobalState {
     struct termios termios;
     bool was_interrupted { false };
     bool was_resized { false };
+    int last_return_code { 0 };
 };
 
 extern GlobalState g;