New name UI_UPDATE for interpret_move's return "".

Now midend.c directly tests the returned pointer for equality to this
value, instead of checking whether it's the empty string.

A minor effect of this is that games may now return a dynamically
allocated empty string from interpret_move() and treat it as just
another legal move description. But I don't expect anyone to be
perverse enough to actually do that! The main purpose is that it
avoids returning a string literal from a function whose return type is
a pointer to _non-const_ char, i.e. we are now one step closer to
being able to make this code base clean under -Wwrite-strings.

unruly.c

@@ -1531,7 +1531,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
     if (IS_CURSOR_MOVE(button)) {
         move_cursor(button, &ui->cx, &ui->cy, w2, h2, 0);
         ui->cursor = TRUE;
-        return "";
+        return UI_UPDATE;
     }
 
     /* Place one */