Bring in proper gmirror support for the GUI and notifications.
Made a general gmirror library to perform various gmirror tasks and get information, using some of the former widget logic to start. Updated widget to use this new code.
Added a Diag > GEOM Mirrors page that displays information about existing mirrors and perform various management tasks. Current actions include rebuilding a drive, forgetting disconnected mirror drives, insert/remove, deactivate/activate, clearing medatada. It's now possible to use the GUI to rebuild a failed mirror by performing a forget, then insert action to replace a missing/dead drive.
Also included is a notification setup. Mirror status is polled every 60 seconds, and if any aspect of the mirror changes, notifications are issues that alert in the GUI and by SMTP, etc.