Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

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

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

Some minor tunings/cleanups inspired by bde@ after previous commits:
- remove extra dynamic variable initializations;
- restore (4BSD) and implement (ULE) hogticks variable setting;
- make sched_rr_interval() more tolerant to options;
- restore (4BSD) and implement (ULE) kern.sched.quantum sysctl, a more
user-friendly wrapper for sched_slice;
- tune some sysctl descriptions;
- make some style fixes.

mdoc: order prologue macros consistently by Dd/Dt/Os

Although groff_mdoc(7) gives another impression, this is the ordering
most widely used and also required by mdocml/mandoc.

Reviewed by: ru
Approved by: philip, ed (mentors)

- sched_4bsd is no longer a default system scheduler on some
architectures, so call it "traditional" instead.

- sched_ule is no longer buggy or experimental (according to
rev. 1.7 of sched_ule(4)), so don't call it experimental
(reported by a user on stable@).

Reviewed by: rwatson

- ULE is no longer buggy or experimental.

sched_core(4) removed.

- Remove SCHED_ULE from GENERIC to better avoid foot-shooting by
unsuspecting users.
- Add a comment in NOTES about experimental status of SCHED_ULE.
- Make warning about experimental status in sched_ule(4) a bit
stronger.

Suggested and reviewed by: dougb
Discussed on: developers
MFC after: 3 days

Revise markup.

Add a manpage for the CORE scheduler.

(Mostly) markup fixes.

Approved by: re (blanket)

Add rudimentary man pages for kernel options sched_4bsd and sched_ule,
which document some of the sysctls available for configuring 4bsd, some
of the bullet features of ule, and that ule is considered experimental
still.

MFC after: 3 days