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

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

Simplify the GEOM OAM api: Drop the request type, and let everything
hinge on the "verb" parameter which the class gets to interpret as
it sees fit.

Move the entire request into the kernel and move changed parameters
back when done.

Add "CONFIG_GEOM" operation to the OAM API.

Collapse meta arguments into regular arguments, the distinction is
more trouble than it is worth.

Run a revision on the OAM api.

Use prefix gctl_ systematically.
Add flag with access perms for each argument.
Add ro/rw versions of argument building functions.
General cleanup.

Start leaking the AOM api into the tree.