Handbook mirroring section

[Warren Block]

Another round of changes, thanks in part to feedback from Benjamin 
Kaduk.  A new section at the start gives examples for erasing old 
metadata that conflicts with setting up a new mirror.  GPT and old 
mirror metadata are the two that have caused me problems.  Are there 
others that should be added?

Still missing is a reference for gzero; is it in a man page somewhere?
The "Boot Problems" section suffers from some lame wording, my fault 
from adding the note about forgetting to load the kernel module to 
Hiroki Sato's original text.

http://www.wonkity.com/~wblock/gmirror/geom-mirror.html
http://www.wonkity.com/~wblock/gmirror/geom.diff