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

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

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

MFC after: 1 day

If NO_LANGCODE_IN_DESTDIR is defined, don't put the language code in
DESTDIR. This avoids redundant information in the path when DOCDIR
points to some directory that already implies (or specifies) a certain
language. This is the case with the web site, where the release notes
are already installed under a language-specific directory. This
behavior is not being made mandatory because it might still be useful
to install all the translations in one directory, such as during
testing, or in a hypothetical release notes archive. Furthermore, it
is not being made the default because that breaks consistency with
stuff under doc/.

Reviewed by: bmah

Collapse the seven *.sgml files containing the content of the README
document into article.sgml. Move the $FreeBSD$ tag from a
comment to a <pubdate></pubdate> entry, but no other content
or whitespace changes.

The two reasons for doing this are to: 1) Get rid of a bunch of
itty-bitty files and 2) help with a reorganization of the README that
will be much simpler if all the content is in a single file.

Add an abstract for the README.

Change the way the value of DESTDIR is determined for the release
notes build. Instead of having doc.relnotes.mk make a guess, hardcode
quite a bit (but as little as possible) in Makefile.inc's sprinkled
strategicly throughout the tree. This has the advantage of actually
working properly (which is a Good Thing(tm)), and the disadvantages of
more files in the repository and more hardcoded paths (which are both
Bad Things(tm)).

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

Build system:
o Define a RELN_ROOT variable which points to the root of the relnotes
tree (i.e., src/release/doc).
o By default, define DOC_PREFIX in terms of RELN_ROOT; this gives a
bigger chance of finding the doc/ tree without help in the form of
setting DOC_PREFIX on the command line.
o Respect DOCDIR; `make install` works now.

Approved by: bmah

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

src/release/doc/README has additional information.

Reviewed by: -current, -doc