Update to r42829:

• general tightening and clarification of content
• the section 3s were changed to section 2s to prevent this chapter from going too deep into headings and to be more consistent with the previous section; I will make igor happy with a white-space change after the rest of the changes to chapter 5 are complete
• listings were updated to latest version of pkgng

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D5410