Implement the `realpath(3)` library function.

Chapter 18, exercise 3.
Implement the getcwd(3) library function.
Show stats of the types of files of a directory using nftw(3).

Chapter 18, exercise 7.
Implement the nftw(3) library function.

Chapter 18, exercise 8.
Add benchmark program of chdir(2) vs fchdir(2).

Chapter 18, exercise 9.