Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Rename the generic "CLASS" to the more specific "GEOM_CLASS".
While I'm here remove redundancy and inconsistencies.

Obtained from: Juniper Networks

- Add a manual page for gmirror(8) utility.
- Connect it to the build.
- Inform geom(8) manual page about it.

Reviewed by: trhodes

Add GEOM_MIRROR class which provide RAID1 functionality and has many useful
features. The gmirror(8) utility should be used for control of this class.
There is no manual page yet, but I'm working on it with keramida@.

Many useful tests provided by: simon (thank you!)
Some ideas from: scottl, simon, phk