daily/223.backup-zfs: improve daily_backup_zfs_verbose behaviour

• 223.backup-zfs would previously honour the daily_backup_zfs_verbose flag for zfs/zpool list, but not for the properties list. fix it to show a diff for both of these if requested.

• if daily_backup_zfs_verbose was disabled, 223.backup-zfs would still set rc=1 if the backup files changed, which caused periodic(8) to send a useless email even if daily_show_success=NO was set.
  
  change this so that it only sets rc=1 if diff output is enabled, i.e. the output is actually useful to the admin.

MFC after: 2 weeks
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1091