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

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

$Id$ -> $FreeBSD$

<bsd.doc.mk> now includes ../Makefile.inc, so don't include it here.

$FreeBSD$ -> $Id$.
Removed Berkeley Id. We never did this like Berkeley (because cvs doesn't
handle symlinks) and no interesting Berkeley words are left.

Create a subdirectory for every document. This make it easier to use
the default formating rules from <bsd.doc.mk>, especially for
PRINTERDEVICE=html.