Process args with while loop.

This will allow easy addition of new options.
Add dry-run option: show what snapshots would be deleted.
Change logfile to /dev/stderr if the dry-run option is on.
Be explicit that it's just dry run.

(Meant to reassure nervous sysadmins like me.)
Merge branch 'prune-dryrun-option' of https://github.com/saintaardvark/adlibre-backup
Improve usage information. Comment and tab->space fixups. Warn if host not found