Rewrite slightly to use the new 6d facedir prediction code in builtin.

Keeps the old behavior of sneak == force wall (rather than invert).

Add protection/ownership checking.  Can be phased out later after protection
mods start taking advantage of the engine's built-in ownership-checking
functions.

Got rid of the /st command, since it didn't work anyway.

Minor re-arrangement of init.lua to put the mod's title block at the top
where it belongs :-)

Adding this will cause this mod to require Minetest from git, commit 1d856b73 or later.