- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

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

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