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

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

add new build knobs and jigger some existing controls to improve
control over the result of buildworld and installworld; this especially
helps packaging systems such as nanobsd

Reviewed by: various (posted to arch)
MFC after: 1 month