flua: move lposix back into flua for now

The real luaposix is structured differently, and our builtin version
tends to model that in its own special way. Build it back in until we
get the shlib model just right to unbreak nuageinit, among other things.

This is a partial revert of the below-referenced commit; the other two
modules are simple, though, and were accurately split out.

Reported by: markj
Fixes: b11a5709ec2b6 ("flua: kick out the remaining builtin modules")