shutdown: let's not use exit() needlessly

shutdown: let's not use exit() needlessly

Generally we prefer 'return' from main() over exit() so that automatic
cleanups and such work correct. Let's do that in shutdown.c too, becuase
there's not really any reason not to.

With this we are pretty good in consistently using return from main()
rather than exit() all across the codebase. Yay!