implement newOrNil, changes to stdlib

- newOrNil works like new, but sets the variable to nil
  if the heap allocation failed
- change stdlib to use newOrNil in openfile and openvolumeid
- changes to programs that use openvolumeid

progs/dumpdir.pas

@@ -14,8 +14,8 @@ var dirs:DirectorySlot;
     error:integer;
 begin
 	lastSlot := volumeTable[volid].part.dirSize - 1;
-	openvolumeid(volid);
-
+	openvolumeid(volid, error); (* we just ignore error here because
+		we should always have enough heap space *)
 	for i := 0 to lastSlot do
 	begin
 		getdirslot(volid, i, dirs, error);