liblua: add loader.lua_path

As described previously, loader.lua_path is absolute path where scripts are
installed. A future commit will use this to build paths for dofile in
try_include, rather than the current pcall/require setup that makes it more
difficult to coordinate loader aborts from local.lua -- we do not need the
flexibility of require(), and local.lua is in-fact not a 'module-like' file
as we will not be referencing anything from it.