David Nickerson reports that it's possible to lay a line over a 'no

David Nickerson reports that it's possible to lay a line over a 'no
line here' cross mark by dragging, and furthermore, that doing so puts
that grid edge into a stuck state that no UI action short of undo can
get it back out of. Fix drags to stop at crosses, and fix execute_move
to fault any move string that nonetheless somehow managed to try to
set a line over a cross without explicitly tagging it 'R'.

[originally from svn r9400]