Added a periodic script to create backups of information for any gmirrors (i.e. the output from gmirror list).
Documented the variables 'daily_backup_gmirror_enable' and 'daily_backup_gmirror_verbose' in periodic.conf(5).
The default for these two new variables have been set to "NO" in the default periodic.conf.
Fixed a comment for 'daily_backup_gpart_verbose' from previous diff review.