Add chapter 23 exercises

@rmascarenhas :smile:
I guess the most apropriate (if this was some kind of serious application and not just an exercise solution) would be to halt program execution, since it would mean something is wrong in the user stack (software stack, not the process stack), or there is a bug in the kernel.

That is because according to the documentation of the `setitimer` function, it should only fail in case you pass an invalid `which` parameter to it (which cannot be the case since you explicitly pass the `ITIMER_REAL` constant), or if the pointers are invalid - which would happen only in a (somehow) broken system.

TL;DR: returning anything here would be fine. :sweat_smile: