Update to r42968:

Another dent in this very large chapter. This patch does the following:

• fixes &os; and most instances of "you"
• fixes manual page repitition
• some word-smithing
• some heading tightening in the NIS section
• some clarification in the NIS server section

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D5303