Test legality even when off if engine defined pieces

Test legality even when off if engine defined pieces

After reception of a piece command XBoard will mark target squares
even when legality testing is off, and now also will require the lifted
piece to land on a marked square (like when the engine highlighted them
through a highlight command).
 A problem is that this does not work for drops, as drops do not mark
target squares. So drops are always accepted. Which of course is bad
for Pawn drops. But there really is no way yet for an engine to specify
how pieces can be dropped, similar to the 'piece' command.