Paired down OS Lua library

Pretty much removed everything except for time operations. File operations steps on the toes of the IO library, and the others were useless or invasive. The only one I wasn't sure about was `os.tmpname()`, but I think that's something that could be implemented for IO library instead if necessary.

The main reason I want this is because os.time provides an OK way to initially seed RNG on game first boot. But it would also be cute to let Lua modders be able to re-implement Christmas mode / day&night cycles themselves :p