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 improvecontrol over the result of buildworld and installworld; this especiallyhelps packaging systems such as nanobsdReviewed by: various (posted to arch)MFC after: 1 month