Rewrite lighting to use engine's 6d facedir prediction code
changed the names of a couple of internal functions to compensate.

this makes lib_6d.lua obsolete.
fix nil refs caused by a renamed function
Fix improper use of translation code in curtains

possible side effect: Some users may have unknown nodes because they
were being registered wrong to begin with.
only register the alias for wrongly-named curtain nodes
if they were in fact wrongly-named.
Merge remote-tracking branch 'upstream/master'