Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC r241096:
- Rename .sgml files to .xml to track the recent doc changes

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Bump version numbers and update descriptions for the 9-CURRENT
world. The %[no]include.historic knobs are removed because they
are not used for a long time.

s/alpha/sparc64/ in examples.

Make it more obvious where to find RELNOTESng source files if someone went
browsing in the old release notes location.

While I'm here, update README files to be closer to reality.

Requested by: obrien
MFC after: 1 day

Fix typos:

* intially -> initially
* accomodate -> accommodate

ISO_* -> ISO* rename

Document s/NORELNOTES/NODOC/ in RELNOTESng README.

Take out the line that says that RELNOTESng will *eventually* become
the release default. It is now.

Reminded by: dd

Update comments on RELNOTESng.

More RELNOTESng infrastructure fixup: The directory layout of
stylesheets (particularly with respect to translations) is now similar
to that of the doc/ tree. Added a customization for HTML page footers
(this is the only reader-visible change).

RELNOTESng infrastructure fixes:

1. There is now only one RELNOTESng stylesheet; the architecture-specific
stylesheets (to handle different values of the arch= attribute) are gone.

2. Several Makefile variable definitions were factored into
doc.relnotes.mk.

Submitted by: dd

Add a note on setting DOC_PREFIX when building release notes.

First commit of RELNOTESng, the rewrite of the *.TXT documentation
files.

src/release/doc/README has additional information.

Reviewed by: -current, -doc