Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

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 gmountver, disk mount verification GEOM class.

Note that due to e.g. write throttling ('wdrain'), it can stall all the disk
I/O instead of just the device it's configured for. Using it for removable
media is therefore not a good idea.

Reviewed by: pjd (earlier version)