Explain necessary permissions for a range of file operations.

Chapter 15, exercise 1.
Explain effect of stat(2) on file timestamps.

Chapter 15, exercise 2.
Check file accessibility based on effective UID.

Chapter 15, exercise 4.
Explain how to retrieve umask without modifying it.

Chapter 15, exercise 5.
Implement the equivalent of chmod a+rX.

Chapter 15, exercise 6.
Add a simple implementation of the chattr(1) utility.

Chapter 15, exercise 7.
Fix typos.
Merge branch 'chap15'