[CMD] Syntax errors during parsing of batch parameters expansion, or FOR and IF commands, are fatal, and batch execution should stop.

- To this purpose use the ParseErrorEx() that correctly sets the
  bParseError flag, and return the partially-parsed command so that
  it gets echoed as well for diagnostics purposes (Windows-compatible).

- Any other parameters specified after (or before) the '/?' switch for
  the FOR and IF commands, are considered fatal syntax errors as well,
  thus we employ the ParseErrorEx() as well.

base/shell/cmd/if.c

@@ -59,7 +59,7 @@ INT cmd_if(LPTSTR param)
         return 0;
     }
 
-    error_syntax(param);
+    ParseErrorEx(param);
     return 1;
 }