Add 6d facedir support

gets rid of all the old "inverted", "wall", "vertical", "top" nodes, adds an abm-based conversion routine to cover them.  Also renamed "bottom" micro/panel nodes.

Also fixed a couple of minor bugs and did a lot of code formatting for
readability (but not mauvebic style ;) )

For now, use the /st command to switch between placing floor/ceiling stairs/panels and the wall direction ones.