Add chapter 18 exercises (still missing custom nftw)

@rmscarenhas  :disappointed: 
You are allowing the dark world of undefined behavior dominate this program :ghost: 

What I mean is that you should check the `flag` passed by `nftw`to make sure you have a valid `stat` buffer. If you happen to have a file or directory to which you do not have read permissions, the contents of the `stat` buffer are undefined, and then all sorts of things can happen. :bird: