Update to r43702:

Rename Amanda to Third-Party Backup Utilities.
Add some examples of utils. This section is light-weight and could probably use another paragraph or two.

Rename Emergency Repair Procedure to Emergency Recovery.
Tighten up this section and put the emphasis on the newer installation media option as opposed to the older livefs option.

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D4167