Update to r43696:

First half of edits to Backup sections.
Merge 18.9 Backup Strategies and 18.10 Backup Basics.
Clean up summary.
Divide utilities into File System Backups and Directory Backups sections (not quite complete yet).
Remove Do Nothing section (we don't want to encourage this).
Remove Which Program is Best section as it is personal opinion and based on 23 year old findings.

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D4121