stand: consolidate knowledge of lua path

Multiple places coordinate to 'know' where lua scripts are installed. Knock
this down to being formally defined (and overridable) in exactly one spot,
defs.mk, and spread the knowledge to loaders and liblua alike. A future
commit will expose this to lua as loader.lua_path, so it can build absolute
paths to lua scripts as needed.

MFC after: 1 week