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)

Tie up some loose ends r88509 left behind:

- chooseproc() is long gone, MLINK choosethread instead
- Update NAME section for choosethread
- Mark chooseproc.9 for removal

PR: 149549
Submitted by: pluknet
MFC after: 1 week

Tabs here upset my automatic checker, so replaced them with spaces.

Update function definitions and required include files to reflect
the current state of the system.

Approved by: alfred

mdoc(7) police: Use the new .In macro for #include statements.

Add 2001 to copyright.

mdoc(7) police: fixed some minor formatting/spelling errors.

Document the current organization of the run queues and the various
functions used to manage them.

Reviewed by: sheldonh