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

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

Fix markup nits.

Options spring cleanup:
- Add and document the KVM and KVM_SUPPORT options that
are needed for the ifmcstats(3) makefile
- Garbage collect unused variables
- Add missing inclusion of bsd.own.mk where needed

Approved by: kan (mentor)
Reviewed by: ru