Rewrite do_castle_move()

And handle the castle directly in do/undo_move().
This allow to greatly simplify the code.

Here the beast is the nasty Chess960 that is
really tricky to get it right because could be
that 'from' and 'to' squares are the same or
that king's 'to' square is rook's 'from' square.

Anyhow should work: verified on all Chess960
starting positions.

No functional and no speed change also in Chess960.