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

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

Add CPU percentage limit enforcement to RCTL. The resouce name is "pcpu".
It was implemented by Rudolf Tomori during Google Summer of Code 2012.

Change the way rctl interfaces with jails by introducing prison_racct
structure, which acts as a proxy between them. This makes jail rules
persistent, i.e. they can be added before jail gets created, and they
don't disappear when the jail gets destroyed.

Rename a misnamed structure field (hr_loginclass), and reorder priv(9)
constants to match the order and naming of syscalls. No functional changes.

Add rctl. It's used by racct to take user-configurable actions based
on the set of rules it maintains and the current resource usage. It also
privides userland API to manage that ruleset.

Sponsored by: The FreeBSD Foundation
Reviewed by: kib (earlier version)