Move shared save/load code in voxed, kwalk, and Slab6 into file shared by all three, and make it work on all platforms

[Ericson2314]

Kind of a two-in-one (re-factoring + porting) issue, but oh well.

[Lensman]

I'll do these splitting tasks first. Voxlap.cpp with ASM as 8000+ lines for instance. We are settting our selves up for trouble using IFDEFS, but we can turn this into a modular structure later. Look at SDL codebase as an example of what I mean.

[Ericson2314]

If we do the big reactor first, I think we should modify the Makefiles so they are a little bit more generic and don't need to be updated with each rebase.