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)

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)

- Stop calling libthr alternative as it's now the default
threading library.

- Now that libpthread is a symlink, it's no longer possible
to link applications with libpthread and have libmap.conf(5)
select the desired threading library; applications will be
linked to the default threading library, libkse or libthr.
Remove an obsolete paragraph.

- Mention that improvements can be seen compared to libkse.

Reviewed by: deischen, davidxu

Markup fixes.

Approved by: re

Remove incorrect BUGS entry: libthr does support process-scope thread
scheduling.

MFC after: 1 week
Submitted by: davidxu
Approved by: re (scottl)

Add a stub libthr(3) man page to document what it is and why, as well as
to point at libmap.conf(5). This will help answer questions about what
and why it is, although not in great detail.

Approved by: re (scottl)
MFC after: 1 week
MFC note: When MFC'd, don't MFC mention of work not yet MFC'd.