Add scheme_atexit c function.

[LeifAndersen]

This way programs can actually call atexit. (Otherwise atexit
is frequently not provided in libc as a symbol.)

[mflatt]

Calling atexit() is not always the right thing. Probably the code in scheme_add_atexit_closer() in "thread.c", which sometimes calls atexit() but sometimes does other things, should be moved into scheme_atexit() and called from scheme_add_atexit_closer().

[LeifAndersen]

Good idea. I have moved the relevant code to scheme_atexit, thanks.

[LeifAndersen]

Would it be a good idea to also bump the version number, or do you think this is too minor for that?

[mflatt]

If you need to depend on it from a package, then bump the version number. Otherwise, I'd just leave it.

[LeifAndersen]

Okay thanks. Finally, do you think its worth documenting on: http://docs.racket-lang.org/inside/Custodians.html?q=atexit#%28idx._%28gentag._582._%28lib._scribblings%2Finside%2Finside..scrbl%29%29%29

Otherwise I think its good to merge. (It doesn't seem like there are any tests for thread.c)

[mflatt]

I think it's worth documenting, and I agree that it's otherwise ready to go.

[LeifAndersen]

Thanks for your feedback. Merged.

[LeifAndersen]

(I've also pushed the docs.)