Easier rotating using screwdriver

I like that you now use facedir to determine sign rotation. But I think using the screwdriver on an arrow sign is very WTF. Luckliy, the screwdriver mod has an API for custom rotation handling. I suggest to simplify sign rotation: Leftclick should rotate sign clockwise while keeping it attached to the wall it is sticking to. Shift+rightclick (with screwdriver) should be ignored.