Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

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