daemon/lua: get rid of __engine symbol in lua

daemon/lua: get rid of __engine symbol in lua

In particular this gets rid of last light user data inside kresd.

It was still causing problems on some systems, for example Debian Sid.
The error was the same: "bad light userdata pointer" from luajit,
but note that the problem can still be triggered by lua libraries,
e.g. cqueues.