Fix generation of hardware.html for 12.0.

- Move definition of various branch-specific variables from
  Makefile to Makefile.inc.
- Include share/mk/doc.commands.mk to ensure ${SVN} is properly
  defined.  If ${SVN} is defined (default) and exists, hardware
  is appended to SUBDIR.  Add MKTEMP to share/mk/doc.commands.mk
  as part of this change.
- If MAN4DIR is not defined (default), use mktemp(1) to create
  a temporary directory to check out the share/man/man4 path of
  the relevant src branch (determined by evaluating the _BRANCH
  variable defined in the Makefile).  Once the dev-auto.end is
  created, the temporary MAN4DIR is removed.  The src-checkout
  and man4-rmsrc targets are limited specifically to exclude the
  make(install) case, as pseudo-repeatable errors have been
  observed under certain conditions that have yet to be determined.
- While here, remove hwlist entities from hardware.xml from
  drivers that are no longer present in 12.0-CURRENT.
- Also while here, bump copyright dates.

This is based on a differential revision that had received no
feedback seven months:
 https://reviews.freebsd.org/D13467

Approved by:	re, doceng (implicit, silence)
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D13467

share/mk/doc.commands.mk

@@ -15,6 +15,7 @@ FIND?=		/usr/bin/find
 GREP?=		/usr/bin/grep
 LN?=		/bin/ln
 MKDIR?=		/bin/mkdir
+MKTEMP?=	/usr/bin/mktemp
 MV?=		/bin/mv
 RM?=		/bin/rm
 ISPELL?=	ispell