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

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

Don't omit ac(8) as part of WITHOUT_ACCT.

The WITHOUT_ACCT switch is supposed to omit tools related to process
accounting, namely accton and sa. ac(8) is just a simple tool that
prints statistics based on data in the utx.log database. It has nothing
to do with the former.

Removed trailing whitespace.

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