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

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

Disable building of pkg_install by default
If people still want to have pkg_install they can specify WITH_PKGTOOLS when building world

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