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

Forgot to hook up gjournal manual page to the build.

Reminded by: simon, piso

Add userland control utility for gjournal GEOM class.

Sponsored by: home.pl