tools - Implement a simple hammer-backup script.

tools - Implement a simple hammer-backup script.

This script operates HAMMER PFSes and dumps its contents for backup
purposes.

It uses mirror-read directive (see 'man 8 hammer') to perform a
dump to stdout that is redirected to a file with or without
compression.

Read /usr/src/tools/tools/hammer-backup/README for more detailed
information.

Also please note the script is not installed with a default DragonFly
BSD installation, so you have to take care of that manually.

Any suggestion or bug report is appreciated!